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Well I am sure the following questions have been asked before, I even tried a half baked attempt to use the search function (without much succes) so I figure I'd start this thread as a means to ask a series of questions regarding the doctrines and operational procedures of various types of units in the hopes of using them in a historically correct manner.

I hope the 'powers-that-be' are willing to shed some light on these mysteries.

ok, first one then;

What exactly was the use of an assault gun? Was it used as low-cost anti tank/armor weapon? Or do I need to assume that the word 'assault' applies to 'assaulting' defensive infantry positions? Where they used as manuevre units or more as a solid force in the center of the attack? In what way did they fit in the combined arms concept, where they an addition to regular tanks or a seperate weapon with it's own infantry support (assuming for a moment they are indeed to assault defensive/fortified positions)?

Many many many more questions to come.


Rho


p.s. Mr Wild Bill? After being chewed to pieces yet AGAIN by one of your scenarios I have come to the point now that every time I see that 'scenario design: Wild Bill' somewhere in the back of my mind that 'Darth Vader' tune starts to play.

I'll take a crack at this one....

It has been my experience that most assault guns were really more of an infantry support weapon. Something like "Really Rapid Response Artillery". Most assault guns have some anti-tank ability but their main use was to soften up prepared positions, dug-in infantry, etc.

awesome, thanks Grimm.
ok the next one..Tank destroyers..although their purpose is pretty clear, how were they used ...as a self propelled AT-gun used to set up ambushes or as a mobile weapon mixed with in the regular tanks during dynamic battles on terrain that really is suitable for such tank battles?

Originally posted by Rhodan:
Well I am sure the following questions have been asked before, I even tried a half baked attempt to use the search function (without much succes) so I figure I'd start this thread as a means to ask a series of questions regarding the doctrines and operational procedures of various types of units in the hopes of using them in a historically correct manner.

I hope the 'powers-that-be' are willing to shed some light on these mysteries.

ok, first one then;

What exactly was the use of an assault gun? Was it used as low-cost anti tank/armor weapon? Or do I need to assume that the word 'assault' applies to 'assaulting' defensive infantry positions? Where they used as manuevre units or more as a solid force in the center of the attack? In what way did they fit in the combined arms concept, where they an addition to regular tanks or a seperate weapon with it's own infantry support (assuming for a moment they are indeed to assault defensive/fortified positions)?

Many many many more questions to come.


Rho


p.s. Mr Wild Bill? After being chewed to pieces yet AGAIN by one of your scenarios I have come to the point now that every time I see that 'scenario design: Wild Bill' somewhere in the back of my mind that 'Darth Vader' tune starts to play.

MINDSPY

There are a number of factors that led to the deployment of STG's as assault guns.

The general preference in the German Arms and Soviet Arms was for the tank.

Given that this was the case the internal politics of the German Army was able to latch on to the procurement of assault guns for non-Panzer Division formations as well as for the artilery and in later dates for armour period. You will find that many elite formations of infantry men were temporary and while they existed they were assigned assault guns.

As well the development of the Panzer formations and the independent battalions and such controlled by Corps or higher excluded the infantry formations from ever having organic STG's in their roles. (unless by special order).

Now some Elite formations such as GrossDeutchLand (sic) were later in the war able to have organic battalions of STG's or assault guns. However this was not the norm!

This is a very quick arrangement of factors leading to the development and use of the STG's. One source of information for the conflict involved in bringing the SP guns and Assault guns within the domain of the Panzer Command can be gained by reading up on the assignment of Guderian to the role of Inspector General.

Essentially tactically the assault guns were assigned to infantry formations particularly when the Germans were on the offensive or attempting to close a gap or other extraordinary circumstance.
In general the assault guns were on the chasis of obsolete tanks and even more importantly lacked good defensive fire.
Being hull mounted guns their aim was time consuming as well the covered arc's depended on mobility and generally they lacked close-in infantry target type firing weapons.
In other words their combined slow rate of fire and poor infantry close assault defense led to their being relegated to the more suitable role of having infantry in support or vice a versa depending on what you read and who wrote it!

This inability of the assault guns to be as effective as tanks did not however exclude them from front line high priority assignments!

They were not however at any time controlled by the German Army's Infantry!

Indeed it is a complex subject where the political office figures as prominently as the Heer (German Army) higher command does as does the politics of industrial command and the assigment of industrail capacity to the conquered or satellite countries!

Very complex indeed but we are only concerned with the game tactics ...

MINDSPY

The Mindspy´s information is perfect.

Originally posted by Rhodan:
awesome, thanks Grimm.
ok the next one..Tank destroyers..although their purpose is pretty clear, how were they used ...as a self propelled AT-gun used to set up ambushes or as a mobile weapon mixed with in the regular tanks during dynamic battles on terrain that really is suitable for such tank battles?

MINDSPY

This one is not so complex.
Generally until the Soviet front opens the Germans will not have to contend with enemy armour much at all!

This is very significant as it not only infleunces army doctrine and training but unit formation as well!

JPz's are essentially found in the op's involving large infantry formations.

It is rare in most years to find them as the dominant or significant ingredient of Panzer formations.
However the Germans are in a continual fight with the start of Barbarossa.
As such you begin to find that they start to put in an appearance as the toll on the main stream tanks rises.
However this is not to be confused with their hierarchy in the grand scheme of things, they are a stop gap measure only when necessary.

Now straight across the board the JPz's of any army do not generally have the role of anti-tank weapon number one! There are the towed guns and artilery for those situations !!!

You will note that the JPz's or tank hunters or SP anti tank guns are only used for the primary role of anti-tank weapons when they are in a formation that is supplied to be so.
Case in point what is the ammo supply of such weapons.
Simply put predominantly HE .

They are expected to aid in knocking out MG nests and prepared positions and such and when necessary other tanks but tanks are not the primary role. - you need to find a formation that receives AP and specialty AP rounds in the main to locate the true anti-tank force. For the Germans this will begin in Barbarossa.
For the allies they will develop some as they meet up with elite Panzer Divisions only.
As infantry are the primary target type in most large scale operations you can guess correctly that these formations of tank hunters were small in number [particularly since the Tanks are out-right preferred by all!]

Tactically any anti-tank weapon is best used in the ambush role.
If by good fortune you have friendly skies and counter-battery capacity then you can fight a more maneuver oriented attack once discovered however if you should lose these aids then ... who is ambushing who is the bigger question!

MINDSPY

Originally posted by Gamael:
The Mindspy´s information is perfect.

Thanks very much!

MINDSPY

MindSpy, glad to see that you mentioned Heinz Guderian. I recommend his book Panzer Leader to any who have not yet read it. If I recall correctly, he deals with the subject of assault guns later in the war. Hitler appointed him too late in the war for his influence to be war-winning. The continued production of assault guns was based on the availability of chassis from earlier tanks and also because the production time of assault guns was quicker, due to not having to install a turret and controls. At that stage of the war the German guns were good and they just wanted to mount them on anything that would move. Had Guderian had his way, they were only intended to fill a stop-gap role on the Russian front. Hope this helps

Originally posted by Randy Stead:
MindSpy, glad to see that you mentioned Heinz Guderian. I recommend his book Panzer Leader to any who have not yet read it. If I recall correctly, he deals with the subject of assault guns later in the war. Hitler appointed him too late in the war for his influence to be war-winning. The continued production of assault guns was based on the availability of chassis from earlier tanks and also because the production time of assault guns was quicker, due to not having to install a turret and controls. At that stage of the war the German guns were good and they just wanted to mount them on anything that would move. Had Guderian had his way, they were only intended to fill a stop-gap role on the Russian front. Hope this helps

MNDSPY

Thank You Kindly.

Yes Heinz Guderian is a significant figure in fact any of the leaders who were willing to use any gun mounted on a chasis vs only what is a prima donna were particularly effective leaders.

it is ironic tha the the Allies insistance on a common design and a mass production of such weapons meant that in fighting a infantry rich formation the Allies would always come out ahead.
However this did not allow for the specialist tank fighter design to come out soon enough. But as Medium tanks save lives (behind 100mm uprated) it is obvious that even less capable designs then the specialist tanks are superior tactics
which is what Guderian and others could immediately recognize!!!

the germans did follow suit on the panzers being streamlined into formations where the significant type could be supplied (Guderians orders!) -- although problem apples like the panther and tiger created the need before the organization was there anyway! by default - their very prescence on the field dictated they should be grouped for supply as much as for combat.

MINDSPY

For a good discussion on assault gun doctrine, I'd recommended "War On the Eastern Front", by James Lucas. He has distilled the knowledge of many sources, both contemporary and post-war, to give a good accounting on various aspects of the war in Russia. There IS a good short chapter on assult-gun doctrine included here. I would also highly recommend "War Without Garlands", a recent book by Robert Kershaw, which is heavy on first-hand descriptions of eastern front combat in the 1941-42 period, again from the German perspective, but has a great description of the siege of Brest-Litovsk in June-July 41, and much tactical detail on other events of that summer and afterwards.

Thank you very much for what appears to be a very well worked out answer Mindspy

A few questions though; ( and although I realize that intially these questions may seem a bit more historically minded then game minded, I do hope to use this information in some scenarios in the nearby future)

1. Reading on both answers you so kindly provided me I can't help but get the feeling that both concepts shared many similarities, most of all the infantry support function in softening up prepared positions ( both seem to share their, intial, dependence on HE )..if that is the case..then why two different concepts? Or did the TD and the AG finally merged into one common assault support weapon?

2. With respect to the doctrine during offensive operations...would I be correct to assume then that the moment a breach had been created in enemy lines the cavalry/armour would exploid that gap, storm through to either 'gain miles' behind enemy lines, with minimal support units, or to cause mayhem directly behind enemy lines, where as the assault guns stayed behind with the infantry to support them in the continuation of the attack? To wrap the remnants of the enemy lines?

Of course my thanks goes as well to the gents who so kindly provided me with titles of various books to read upon.

Rho

The politics of assault guns was alluded to earlier. Specifically they were artillery branch units, not armored troops. When Guderian (I think it was he) was appointed inpector of armored troops, he did not have authority over assault guns. So while they may have had a tactical role (infantry close support weapons that were cheaper to produce & maintain than a tank, no turret for example), much of their existance is due to political rather than militay roles.

Originally posted by Rhodan:
Thank you very much for what appears to be a very well worked out answer Mindspy

A few questions though; ( and although I realize that intially these questions may seem a bit more historically minded then game minded, I do hope to use this information in some scenarios in the nearby future)

1. Reading on both answers you so kindly provided me I can't help but get the feeling that both concepts shared many similarities, most of all the infantry support function in softening up prepared positions ( both seem to share their, intial, dependence on HE )..if that is the case..then why two different concepts? Or did the TD and the AG finally merged into one common assault support weapon?

2. With respect to the doctrine during offensive operations...would I be correct to assume then that the moment a breach had been created in enemy lines the cavalry/armour would exploid that gap, storm through to either 'gain miles' behind enemy lines, with minimal support units, or to cause mayhem directly behind enemy lines, where as the assault guns stayed behind with the infantry to support them in the continuation of the attack? To wrap the remnants of the enemy lines?

Of course my thanks goes as well to the gents who so kindly provided me with titles of various books to read upon.

Rho

MINDSPY

Now before we go on much further I would recommend the read ... (KG Erwin).
So that after the read we can have some more questions on how what where and we could probably start again with the postings.

I know that the topic is complicated as there are political reasons for non-effective tanks in all the armies (principal players) as well as tactical considerations and of course the internal politics of the armies in question so certainly a posting will not do justice to the topic, particularly when certain ideas are not yet settled in the mind.

So I will take my time read up some more then return with more recent insights to clarify the mix somewhat of not only mutually contradictory circumstances but policies and tactics and organization as well.


Now to answer as best I can your question (1) (2).

The use of the words Assault gun and Tank Destroyer is fraught with difficulties.
Like any history it matters greatly who wrote what you are reading.
The specialist? The historian? the joe blow soldier or commander from the period.
Do I use German ranks or their so-called euquivalent US Army ranks or the Brit's and so on ...

The use of the words Assault gun and Tank Destroyers should not be used inflexibly or to denote the specific use of a weapon or to even state it's type. "Many quotes on why this is -from hiding units in formations to just simply needing to present a picture for higher echelons regardless of whether or not this is an accurate description of the weapon type-yes lying for the bigger picture-at their own detriment"

Such it is on the subject of these weapons.
They are referred to as both and not, yet, the same models are often the subject of the discussion.

So let's do a better job then the written record and just refer to function then.

Hull mounted AFV's:
They are used with supporting infantry/to support infantry much like the heavier tanks because of their slow rates of fire.
This is more for their own protection and the sheer size of the Soviet Army then for tactical reasons.
They can be very easily overwhelmed.
The hull-mounted guntype is not the only assault gun possible.
In times like this it is simpler to go by the german word for the MKiv anti-infantry type and the STG 105 mm or 75 HE type!
Still in English the confusion reigns.

Mopping up.
The german army does not have an oversupply of STG's or other weapons on a chasis. In fact most of the mopping up is accomplished with infantry and artillery in support plus the less capable/less experienced air units that can often be found in aiding the mopping up situation.
It is however important to not get caught up in the use of formations since availability can result in tactics and uses that are not the norm but in reading a topic these differing instances appear to hold much merit. <<Having the tapes is more important then the reason for such tape making procedures. So "information" - we will have to work and re-work you until a predictable pattern emerges.>>

On the offense.

The preferred method for the German Blitzkrieg is to use motorized-mechanized-and
armoured units in the ASSAULT.
Infantry do not play a major role in an offensive for the Germans. To achieve a breakthrough the Germans use the wheeled units almost exclusively this is not just doctrine it is the only plans held up to serious scrutiny by the German higher command! Herr Adolf included.
(Physics alone guarrantee's that leg units should only follow the attack!)
Now in game terms or in the historical--
Armoured units are typically slow, it is in fact the motorized or (mounted-halftrack-mechanized) units that are primarily infantry that you use to get the major exploitation ranges -- the lightly armoured units! I am not saying that you let the trucks in convoy formation lead-the exact opposite.
As well since armoured units eat up gas in prodigious amounts there is a further limit in how far they can exploit -- to boot when in laager for resupply, eating and other things such as rest, they were vulnerable to the foot units hence the need for each formation type to be in close support of one another.
This resulted in the rarity of purely armoured units in the German Army!
This lesson did not need to be learned much by the astute - however they are not the single embodiment of any army so many errors can be found time and again in the deployemnt of pure tank or assault gun type units and their failures.

So to prepare for a German offensive.
Round up all the available tanks in the corps
similarly for all SP arty and other gun types.
Scrounge for all available half-tracks and trucks for the mounted infantry.
Assign air units to forward bases so multiple daily sorties are possible.
Assign air units to attack formations and targets in advance of the main armoured and mounted forces.
Assign dug-in Corps arty to the inital barrages!

These things must all be done in unison to depict a typical German Blitzkrieg.
Leave out anything and you do not have the clear operations picture! (or we are studying a less capable commander's organization of forces).

INFANTRY::On the Assault: it is not the preferred use of a formation for the Germans!
They are slow - too vulnerable - they will not gain the 450 mile plus penetrations expected for a Blitz type operation.

Yet the main target type for all offensive operations will decide the main shell type and simply put HE is number one.
For all AFV types HE is number one.

Certainly tanks will be met. But in all cases it is the need to defend against the infantry that is paramount since AP does not carry the punch required.

So as time goes on and the troops encounter the greater sized formations of Soviets tanks of all types.
The AFV's with AP are sorely missed.
Within the Infantry army the terms of tank destroyers carry the meaning -- I won't have to close assault a tank.
But the types used vary greatly.
thin skinned HE types and not.

TD or AG. These new titles are both proper and insignificant. !!!!!!!!!!!!!!!
HE heavy caliber gun or light: to Long bore AP type guns with HE still possible.

For the germans no new supply of specialist anti-tank shells will be delivered to units only the standard AP types flow hence what is the difference between them then if they still have the same ammo type and ...
(However corps controlled supplies of such rare AP special rounds can often be shipped to a unit that is expected to lead or defend against an enemies massed armour.-this is by high command control.)

Going back to the TD vs AG problem.

Now the Allies do not have a specialist tank hunting armoured unit on DDay nor will they for quite some time.

On the other hand they will not face German armour in the numbers of the Eastern front so that in itself excludes the formation of these so-called TD units being in the majority! However they are needed and many political reasons are given for the way in which this subject was waffled on by all the Armies!
Late in the war we start to hear of TD units.


Blah blah

The subject is filled with contradictory information but fortunately it isn't until you discuss the Allies late in the war and see the difference in ammo supply for the favoured anti-tank unit (hull-mounted gun or not) in all types of shells that the picture starts to take shape.

Tank Destroyer units are better supplied in tank destroying ammunition then the majority of regular units (they are purpose built and assigned-to blunt the enemies armoured units in an advance or delay). They are usually commanded from Corps!

That is about the best that we can do.
It is a result of the AFV's fulfilling both missions - anti-infantry and anti-tank that -
we have the conundrum of what are we talking about.

With this comes the problem of what to call them exactly well before the historical designations catch up.

And what language to use while doing so!

MIDNSPY

Thank you Larry for that informative piece of info re those ag's being under the command of arty units, that really was new to me.

Mindspy, once again thank you for taking the time to make such an in depth and elaborate reply, am I safe to assume then that it was not so much the chassis that decided wether an AFV was a dedicated/specialised, but more so the amoutn of ammo it carried with it?

As for the Allied TD's...looking at the encyclopedia of SPWaW I see the M10 Wolverine as been avaiable from 1942 till 1949 with APCR ammunition...this has been a sacrifce the game made in favor of playability?

MINDSPY

Before I begin
Uh no the Arty never controlled tanks or other armoured units ... :<<those ag's being under the command of arty units, that really was new to me.>>

The use of obsolete German and captured chasis to place indirect fire guns (usng HE)
begins in earnest in the fighting during and after in France. There is now a greater supply of chasis for these early SP Arty guns. It does not receive universal acclaim but those in the know realize the potential for BlitzKrieg.
Many years later when Guderian is put in charge there is a problem. If his control is to be across the board then many units will lose their SP Arty to Panzer commands. There aren't a great number! However those with the power immediately fight this ... now Guderian has the problem of certain types of SP Guns falling under the definition of
"those unit and weapon types not under his control". It is understandable this would occur as not one of the most senior German Army commanders is a Pz Army - Blitzkrieg type - originator nor even apostle! It is a war of words where more is claimed to be under the blanket definition then is really intended by the artilery arm and others.
It is imperative that this be worked out which Guderian achieves while stepping on some toes.
That does not mean that the STG tanks or other similar tank types or hull mounted tank types were controlled by the artilery arm at any time.
It was instead a response to Guderian's assignment and the definition of his role answerable only to Hitler in the chain of command that caused the artilery arm to quickly attempt to protect their assets ...
Afterall after a few retreats it is easy to see that all that remains of some artilery formations are the SP types on obsolete chasis of earlier tank designs and to lose them internally would only add insult to injuries already sustained at the hands of the Soviets. (Even Guderian acknowledges the reasons why the definition of what Guderian could command as Inspector was challenged--
Guderain did not lose and neither did the artilery and a compromise was worked out and the Pz Div's never lost control of the STG and other types and in the end the Pz Div's were reformed with organic SP Arty units!)
As was intended.

Now in Guderain's book he uses that particular style of language that is known as hyperbole or exageration which depending on your point of view either makes Guderian the incredible guy that he is or putting one and one together that he is lees adept at being diplomatic that is desired especially for an historic memoir! many statements and their conclusions are rather shall we say do not necessarily follow although the events and dissagreements may be factual his written record is not the last word !
Now I was 12 years old when I read his book so I didn't ahve the finesse of being older to notice this so gradually you come to realize that in the written record of Guderians book is found also his prejudices and to some degree the emotional tone of the situation can be seen -- which tells us that we should allow for Guderain's style of presentation and not go so much for the direct quote as being the last word.

Your first query:<<am I safe to assume then that it was not so much the chassis that decided wether an AFV was a dedicated/specialised, but more so the amoutn of ammo it carried with it?>>

-tanks were preferred by all nations!
-the Germans used the obsolete tank designs of captured weapons to place their guns on.
-both tanks and these guns on appropriated chasis were, as well as older German tank designs like the Pz 1 and 2 using the same guns 75mmHE type with the short barrel and the longer 75mm guns as well as other combinations of gun types. But they are not called AG or STG or TD on the basis of their chasis or gun type.
-furthermore the elite category of TD or AG comes from the way in which corps decides to use you and what they have planned for you and what they WANT to call you.
Relevance to these titles in the form of ammo availability is not a requirement.
(in essence I could fit you with miserable tank vs tank fighting types and call you a TD unit! This is not only historically accurate but quite simply the case ... however if you are more serious and precise in what you want to depict then it is essential that you recognize that only late war Allied units (US/British/others) will be in the category of having their special ammo types replenished after use and that invariably these units were under Corps control as they are needed where the bigger picture demands them and the supply of the ammo being so limited cannot be used across the board by any unit in the chain of command.

Your other query:<<As for the Allied TD's...looking at the encyclopedia of SPWaW I see the M10 Wolverine as been avaiable from 1942 till 1949 with APCR ammunition...this has been a sacrifce the game made in favor of playability?[>>

-having ammo available is not the same as being THE unit that receives this ammo on a priority basis.
-there is little doubt that if your unit commander or supply officer hoards these special ammo types then the probability that you have access to these ammo types does exist -- that is not the same as having the special ammo types as being available for use
and your supply replenished as you consume it. This applies straight across the board to all armies.

-undoubtably there is a limit to what the game can give you in select your own forces however if you use the scenario editor and keep handy your source you can create relatively accurate unit types and their ammo supplies you can depict by increasing or decreasing the types in question.


You are slowly being sucked into a problem that is created from many areas, not the least of which - also complicates the problem - is that the majority of factors do not stem from the battlefield!

The German Army like most active armies has great difficulty keeping it's general staff officers and commanders progressing through the ranks if they are from frontline unit types.
Hence there is a disproportionate number of
rear area formations being promoted over the years to overall command.

Artilery officers and other echelons such as supply have a greater survival rate hence ...

This is important as the Prima donna's of the wartime German Army in WW2 were not Infantry or Artilery officers - they are instead the ones who spoke fluently the strategy of using armoured and mechanized and motorized troop formations in the attack but they ARE commanded by now General Staff officers who were arty or supply or other and the relatively smaller surviving commanders from leg units!

The point being then that the Prima Donna's are upstaged or interfered with. Along comes Guderian the tacit need to promote commanders with battlefield experience and further to
focus on the Armoured capacity of the German Army is at last recognized at the highest levels of the German Office.
The political office (mainly Hitler was already aware of the significance of the armoured unit types but this is not so across the board).
Hence the battle within the political and military for how things should be done!
Add in the complexity of the Luftwaffe and you have a serious problem at the highest levels on where to foucs Industry and for how long!
Afterall building things that entail loss of life will not endear you to the population for long!!!

MINDSPY

Originally posted by MindSpy:
MINDSPY

Before I begin
Uh no the Arty never controlled tanks or other armoured units ... :<<those ag's being under the command of arty units, that really was new to me.>>

The use of obsolete German and captured chasis to place indirect fire guns (usng HE)
begins in earnest in the fighting during and after in France. ... If his control is to be across the board then many units will lose their SP Arty to Panzer commands... now Guderian has the problem of certain types of SP Guns falling under the definition of
"those unit and weapon types not under his control". ...
That does not mean that the STG tanks or other similar tank types or hull mounted tank types were controlled by the artilery arm at any time.MINDSPY

You comment on two types of units, SP arty and direct fire STUGs. I did not mean to comment on the SP types but only on the STUGs. I did not mean to imply that Stugs were ever under artillery unit control in TO&E units but that they were manned by artillery branch troops and the "program" (design decisions, funding, etc.) was under the artillery branch. This had two implications. The artillery branch stressed gunnery so the STUG crews had good gunnery training, at least until the war went so far downhill that training standards went down too. Also, the whole concept of hull-mounted direct fire HE vehicles survived not just because of tactical reasons but because the artillery branch was not willing to discontinue them and give up part of their bureaucratic empire, funds, etc.

I hope this clairifies what I meant to say.

ok..considering they were not (all?) in a direct line of command for the panzer commanders (at batallion/regiment level from what I understand) they were then attached to support a certain tactical operation after which they were returned to their 'pool'?

Speaking in game terms this would mean that a unit of AG wouold not realistically be part of the same core unit that fights it's way through a campaign, but rather an AUX unit purchased for the duratation of one maybe two ops?

As for the confusing part, don't worry too much about it..I find the politics in higher levels command as interesting as battlefield decisions.

Rho

Here's my 2 cents on Assault Guns and Tank Destroyers:

Germany built a lot of vehicles with hull mounted guns in order to make use of obsolete and captured hulls, the USSR did much the same thing in a smaller way to mount big guns in small hulls (SU85 and SU100.) You can mount a significantly larger gun in a hull mount than in a rotating turret. It is also easier and less expensive to make the hull-mounted vehicle than to make a tank. By mid 1943, hulls such as Pz2, Pz3 and Pz 38t were too small to hold useful guns in turrets, but they were large enough to hold the very useful 75mm L43 and L48 guns in a hull mounting (Marders, StG 3 and Hetzer)

The US was the only country to use significant numbers of turreted tank destroyers. These had the benefit of mounting larger guns than could be mounted in tanks of the time (yes, I know they eventually figured out how to mount the 76 in the Sherman, but they couldn't do it in 1943.) Unlike vehicles with hull mounted guns, the American TD's were as expensive and complex as tanks, which is why only the US built them.

A lot of good points have already been made about the doctrine and useage of these vehicles. I'd just like to add that the commanders in the field used them as needed and as they saw fit. I'm sure there was nothing in US TD doctrine about using M10's to take out bunkers with close range direct fire, but thats what they did in the Huertgen Forest.

I raise the anty and here is my two bits

Sturmgeschutz - Assualt gun
Panzerjager - Self-propelled Anti-tank Gun
Sturmartilerie - Self-propelled Guns

These are not the same, although the same vehicals were used in more than one of these rolls.

Please excuse the long quote:
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"Another indication of the neglect of the mechanised force that was apparent despite the doubling of the numbers of its divisions, lay in the increased use made of the Sturmgeschutz (assault-gun). This machine was the brainchild of von Manstein, who envisaged it as a self-propelled, close-support gun for the infantry divisions. It was designed around the same chassis as used for the PzKw III, and mounted a short-barrelled 7.5cm gun within a well-armoured superstructure. The major difference between this machin and a tank lay in its lack of a turrent with an all-round traverse, which prevented it from dealing with the unexpected situations that continually occur during attack. Design work was initiated in 1936, and production begun in 1940. For the attack on France and Flanders, three batteries, each of six assault-guns, had been formed, athough only one battery was ready for action by 5 May 1940. At the time of the invasion of the Soviet Union, however, there were eleven battalions (each of three batteries) and five independent batteries of assault guns; a total of 250 assault guns, out of a stock of some 390, took part in the initial offensive.
To the panzer leaders, the assault guns were unwelcome. Nor was it simply that these new machines did not behave exactly like tanks; there were more important reasons. First, they objected to the whole principle behind the assault-gun development: that of infantry support. In effect, the establishment of assault-gun battalions was simply a reversion to the principle of panzer brigades designed to cooperate with the infantry divisions, against which Guderian and his supporters had fought so hard before the war. Second, the assault-guns came under the control not of the panzer arm but of the artillery, which meant that there was now an expanding armoured force that was not subordinate to the panzer command, but was, instead, the protege of a rival, and a conservative, arm of service -- one, moreover, that was ambitious for recognition in the field. Finally, by mid-1941, the output of assault-guns amounted to as much as one-fifth (some fifty per month) of the output of tanks. This meant that scarce production facilities and raw materials were being expended on what they considered to be harmful diversion,instead of being concentrated on building up stocks for the badly equipped panzer force."

'The German Army 1933-1945 Volume II: Conquest' pg 186-187 By: Matthew Cooper.
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Now as the war progressed the Germans realized that they lacked enough mobile AT guns so some models the Sturmgeschutz were upgraded to long guns to double as mobile AT. These could be used in assualts as originaly intended AND be used as mobile AT guns in defence. So the Sturmgeschutz crossed the line between assualt-gun and Panzerjager(Self-propelled Anti-tank Gun).
The ultimate Sturmgeschutz is the StumTiger.

coming upon this link today after followign a trail started by a URL elsewhere...
http://www.feldgrau.com/pnzfwd.html

in the light of the curent discussion pay attention to pts 21, 23 and 24