AP ratings in indirect and direct fire applications

[cathar1244]

In looking the equipment file, I am confused by some of the variation shown in the AP rating for given weapons -- I mean the variation shown between their direct and indirect fire use.

The period in question is mid-20th century. Following are some quotes from file. The first figure is the direct fire rating; the second figure is the indirect fire rating.

Note that the direct fire AP rating for tanks includes a machine gun's effect. This effect (either +12 or +27 to the rating) has been subtracted out in the data below.

Code: Select all

 75mm		66		90
 90mm		150		96
 100mm		196		116
 105mm		128		128
 122mm		179		179
 128mm		196		196
 150mm		260		260
 152mm		220		220
 

What is interesting is that the final five sets are identical for both applications. And this makes the first three really stand out. Why the variation between the direct fire and indirect fire rating?

Cheers

[cathar1244]

Bump.

Still find this one odd.

One addition is the 105mm gun on the M60 tank. With the MG contribution taken out, it rates as 203 vice the 128 AP rating of a 105mm indirect fire piece.

Oddest bit is at 75mm and below, the direct fire pieces are penalized in terms of AP rating. A strange inversion of values.

Cheers

[golden delicious]

AP strength is always a product of shell weight and rate of fire. This likely answers for any variations, as in practice a 75mm tank gun and a 75mm artillery howitzer are quite different weapons even if the shells are the same size.

[cathar1244]

AP strength is always a product of shell weight and rate of fire. This likely answers for any variations, as in practice a 75mm tank gun and a 75mm artillery howitzer are quite different weapons even if the shells are the same size.

Yes, but ...

In the system noted above, direct fire weapons above 75mm are more effective than their indirect counterparts, while those 75mm and below and LESS effective than their indirect fire counterparts.

That is an odd inversion, if only because direct fire of HE shells produces a less effective fragmentation effect than projectiles fired indirectly. Why would projectiles above 85mm suddenly be more effective than their indirect fire counterparts? Particularly in the confines of an armored vehicle ... that should lessen the rate of fire, not increase it.

Also, I am not convinced that the AP rating is the product of the shell weight and the rate of fire. That has been put out as a "rule" forever, but there is more to the ratings than that. Even Norm, WAY back when, mentioned shell lethality as one of the inputs to determining the AP rating, and weight does not equal lethality.

Cheers

[Lobster]

Maybe there's a concussion value due to the larger amount of explosives not to mention more of the shell to fragment. So a heavy shell would have more of both compared to a lighter shell? Just guessing.

Are both values for the 75mm howitzers or guns?

[cathar1244]

Maybe there's a concussion value due to the larger amount of explosives not to mention more of the shell to fragment. So a heavy shell would have more of both compared to a lighter shell? Just guessing.

Are both values for the 75mm howitzers or guns?

Hi Lobster,

For 75mm pieces, if they fire indirectly, the AP rating is 90. If they fire directly, it is 66.

I got a better understanding of it yesterday.

It is muzzle velocity that is the driver that alters the rating for the direct fire pieces. My guess is that MV is used as a way of giving a bonus (or penalty) to the AP rating to reflect increased or decreased accuracy.

Cheers

[Lobster]

I pay no attention to modern equipment so I don't know the answer to this. How far beyond WW2 do these differences last? I'm just wondering if more modern targeting makes a difference.

[cathar1244]

I pay no attention to modern equipment so I don't know the answer to this. How far beyond WW2 do these differences last? I'm just wondering if more modern targeting makes a difference.

Not tied to targeting. There are two versions, for example, of the 85mm antitank gun. One has "optics 1", the other has none. They both have a "promoted" antipersonnel rating.

I think the "why" is that with larger (bore diameter) guns, they have larger propellant charges for the shells, and the shells have a higher muzzle velocity.

You can check stats if you're curious. World War II 75mm gear had around a 550 meters per second muzzle velocity (for high explosive antipersonnel shells). Postwar, as the guns were larger, they had a MV between 700 and 900 mps.

As far as how far beyond WW2 do the ratings difference last, the answer is pretty to the most recent equipment. AT guns fell out of favor in what, the 1950s? But the tanks in TOAW also have the "promoted" AP rating as their muzzle velocities, even for antipersonnel projectiles, are significantly higher than those in World War II.

Cheers