German 75mmL43, L48 and L46 velocities

[Yoozername]

Using the assumption that the KWKL43 and PAK40 both fired the same AP39, had similar rifling, and that the actual distance in projectile in bore was the same, one could make a linear rough estimate of the velocities from the powder charges.

(2.5Kg/2.7Kg)*792m/s= 733.33m/s

This is 1% of the often quoted 740m/s velocity for the L43 weapon.

I am actually believing that the L48 weapon was actually in the 770-790 m/s range once the ammunition issue was worked out and the 2.5Kg powder was put back in.

[Mobius]

ORIGINAL: Yoozername
There is some interesting information in regards to the L43 and L46 barrels. There is evidence they may actually have been the same.

Interesting theory
From Aberdeen data:

75mm Pak 40 (L/46)
Length of ordnance (incl. muzzle brake): 145.67” (3700mm)
Rifling length: 96.85” (2461mm)
Length of chamber: 28.19” (716mm)

75mm KWK 40 (L/43)
Rifling length: 97.12”
Length of chamber: 20.03”
Length of bore: 117.15”
Length of piece: 126.6”
Additional Length of muzzle brake: 12”
Rifling: Increasing RH 1 in 29.39 to 1 in 19.84

75mm KWK 40 (L/48)
Rifling length: 112.12”
Length of chamber: 20.03”
Length of bore: 132.15”
Length of piece: 141.6”
Additional Length of muzzle brake: 12”
Rifling: Uniform RH 1 in 25.8

I tried to add things up for the L46 and L43 to get the same dimensions other than the chamber but come up a little short. Close but no cigar.

It looks like bore + chamber + 9.45" = length of piece. LoP/2.95 = L#. I don't know what that 9.45" is.

[Yoozername]

From the British Chobham report on a captured Panzer IV G

[Mobius]

Kind of odd. I looks like the muzzle brake would account for 40.5 cm or nearly 16" yet 139-127 = 12". ?

[Yoozername]

From the Chobham data, the length of piece (minus muzzle break which is 12") is 127 inches. That is 3226mm. Divide 3226/75=43. Almost assuredly, the L43 comes from this.

Note the rifling length is 97 1/2 inches. It is reported as 97.12 in the Aberdeen data. The L46 is reported as 96.85 inches in the Aberdeen data. Extremely close lengths of rifling measurements.

Length of ord. for the L46 includes the muzzle break. So, 145.67 minus the muzzle break (assume 12 in.) is 133.67" x 25.4/75mm= 45.27. Close enough for L46.

133.67-127=6.67 inches.

(to be cont.)

[Yoozername]

Wow, it never stops. According to some sources, there appears to be a longer StuK shellcase. I have the length of the PAK40 complete 7,5 cm AP39 round (969.2mm) and its shellcase (Patronenhülse) is 714mm. While researching the KWK/StuK related dimensions, I came across this. My primary source states the KWK/StuK 40 should be 495mm case. But...

7,5 x 49,51 Patronenhülse (6339) der 7,5 cm KwK 40
7,5 x 51,4 cm Patronenhülse (6310) des Stu.G. Ig 7,5 K

[Yoozername]

Some notes:

Muzzle brakes typically overlap the actual barrel. I would assume they would be threaded on and might even have some sort of adjustment. There were several versions for the 75mm and they got progressively tighter as far as the dimensions of the two rings the projectile went through.

It looks like bore + chamber + 9.45" = length of piece. LoP/2.95 = L#. I don't know what that 9.45" is.

This does not work out for the L43. If I follow you,

Rifling length: 97.12”
Length of chamber: 20.03”
Length of bore: 117.15”
Length of piece: 126.6”

Bore+Chamber=137.18/2.96=46
I don't see how you add the 9.45"?

OK, if you look at the Chobham info, they list Depth of Breech opening. 9 1/2 inch. It's the difference between the Length of Piece and Length of Bore.

[Yoozername]

The Length of Chamber matches the Patronenhülse very closely as far as the Pak40 and the 495mm KWK/StuK 40 cartridge length. So the actual projectile length that is protruding inside past the breech would be projectile lengths that seem equal. I would assume these lengths would be similar for like projectiles. A Pnzgr 39 would stick into the unrifled section of the barrel of each weapon equally.

It would seem that the PAK40 would have slightly more barrel travel then.

[Mobius]

Oh, sorry for the confusion. I meant rifling and chamber = bore.
Bore + 9.45 = length of piece.
The L43:
97.12+20.03=117.15 117.15+9.45=126.6 126.6/2.96=42.87

[Mobius]

ORIGINAL: Yoozername

Wow, it never stops. According to some sources, there appears to be a longer StuK shellcase. I have the length of the PAK40 complete 7,5 cm AP39 round (969.2mm) and its shellcase (Patronenhülse) is 714mm. While researching the KWK/StuK related dimensions, I came across this. My primary source states the KWK/StuK 40 should be 495mm case. But...

7,5 x 49,51 Patronenhülse (6339) der 7,5 cm KwK 40
7,5 x 51,4 cm Patronenhülse (6310) des Stu.G. Ig 7,5 K

Did you notice while looking up old Rexford posts that there was one that gave a different dispersion value for the 75mm/L48 tank gun than the one on the StuG. I thought it odd at the time.

Yes, the experimental L40 had a complete round length of 794mm.

[Mobius]

One thing more. In Spielberger's Sturmgeschutz book it covers the development of the 75mm for the StuG. Three test L/46 guns to be produced by Rheinmetal-Borsig. The production plans list them as L/46. The nomenclature of the longer gun was changed to 7.5cm Kanone 44 L/46 to 7.5cm Sturmkanone 40 L/43 on March 16 1942.

[Yoozername]

I think I do recall seeing L43 dispersion values(?) in Rexfords posts. It's easy enough to search the BF posts. I like to look at sources and even the Panzer IV Chobham info has that mysterious gun drawing with metric values.

The raw dispersion values are just one weapon characteristic. Even if the tungsten dispersion is slightly higher, like the L48 AP40 rounds are, the actual battlefield accuracy is still better than the AP39 rounds due to a flatter trajectory and range-error 'forgiveness'. If the AP40 rounds are used with an SOP like "Range must be 1000 meters or less", then they would be clearly superior as far as hitting (and most likely penetrating).

Battlefield range estimation would depend on the situation and weapon type. 88MM FlaK, and other FlaK and artillery, did indeed have range measuring, not estimating, equipment. Even infantry Heavy Weapons companies had similar equipment. In a defense, most weapons could be tied in to the terrain as far as range markers and range cards.

But that is really another topic.

[Mobius]

The comparison is more interesting. Because of the higher dispersion the APCR is less accurate at close range where it is more effective. Because of the flatter trajectory it is more accurate at long range even though its penetration really falls off.
Here is a side by side comparison with 18% first shot range error.

[Yoozername]

Not sure that jives with the data:

75L48 TUNGSTEN CORE
100m....0.1m/0.0m
300m....0.2m/0.1m
500m....0.3m/0.3m
800m....0.5m/0.4m

75L48 APCBC
100m....0.1m/0.0m
300m....0.2m/0.2m
500m....0.3m/0.2m
800m....0.4m/0.4m

Out to about 800m, there is very similar dispersion difference. At these ranges, one could 'zone-fire'.

[Yoozername]

ORIGINAL: Mobius

This table answers some of Rat's questions. It is also in the link I provided and worth downloading.

The actual angle super-elevation for this weapon for targets at 1000 m is 7/16 of a degree. The impact angle is 8/16ths of a degree. That is half a degree.

Drift is negligible since drift, a function of twist and projectile spin, is related to angles of elevation and the gyroscopic forces when the projectile tilts over in flight.

[Yoozername]

I was emailed and told that Rexford's book has a brief mention of powder weights in regards to German 75mm. I checked and it does. But it is a wrong weight and as usual, he seems to like to throw a theory or two into the mix. In any case, he never seems to hit upon the idea of powder wieght being possibly important to armor penetration.

From my current readings and investigations, it seems the Germans had a somewhat unique way of testing and proofing and finalizing designs for powder weight and resulting velocities. In a nutshell:

Germans decided on a AP design and gun. They then decided on a penetration criteria and then set out to get that penetration by varying the powder weight.

The method was to fill the cartridge and shoot it and measure the velocity and see what penetration result there was. The Germans used high quality test projectiles and the goal was to detrmine the full success penetration's velocity and also the full protection velocity (no penetration). Once they had 5 complete penetrations, the test had found the correct velocity, and then the production version would include 5% more weight powder!

This is interesting in that it can explain the variance in supposed 7,5 cm KWK/StuK powder weights. Some sources claim 2.41 Kg powder weight while sources like the manuals and even dug up WWII rounds show a higher 2.5 Kg weight. It is quite remarkable that 2.41 x 1.05 =2.53 Kg.

It is very interesting that the Germans firmly believed in full penetration and the mechanism of HE burster. But, even they admitted that they had big problems with these bursters and especially when attacking sloped armor. The AP penetrators would experience high forces when penetrating due to turning and teh dud rate was significant.

[Mobius]

While theoretically the test needed to have 5 of 5 complete penetrations with intact burster there was a practical problem. That was they had to know at what level the partical penetrations occur. I mean they could of fired an 88mm/L71 at a 70mm plate when it could penetrate twice that. That would of produced 5 or 5 penetrations but would be wrong. So they had a formula to get their final value which took into account both partial penetrations and full penetrations.

My solution would be to find where the 50% level was like the US Naval Limit and then move three std deviations more. But, alas they had other ideas.

[Yoozername]

No, that is not the methodology. They had a system of increasing velocity from non-penetrations to various states of penetration till they achieved the full penetrations. They required the five penetrations to be in a narrow velocity band.

They actually figured out the full protection first. That is, they shot the gun so that they could find the velocity that achieved no penetration results. It was called 'eye-ball' safe behind the armor.

The Germans actually stated the increased performance needed for production rounds to be 5% more velocity. So, if a test velocity showed that 740 m/sec satisfied the penetration criteria, then the production velocity should be 777 m/sec.

[Mobius]

ORIGINAL: Yoozername
No, that is not the methodology. They had a system of increasing velocity from non-penetrations to various states of penetration till they achieved the full penetrations. They required the five penetrations to be in a narrow velocity band.

All 5 had to have limit velocities within 30 m/sec.

[Yoozername]

What version of the Rexford book do you have? He idscusses this and I think he makes a gross error in his conversions.