Silly AT gun question

[noxnoctum]

How is it that a 57 mm AT gun will penetrate where a 75 mm gun on a tank won't?

Is there another factor I'm overlooking? Is it simply length of the barrel that gives it more velocity or something else?

[costard]

Not at all a silly question, noxnoctum. Not a trivial question either.

Let's see... umm. Mostly you've got the answer - the higher velocity of the 57mm gun gives better penetration than a lower velocity 75mm gun. But a 75mm projectile with the same velocity as the 57mm will have better penetration (for a projectile made of a given material):

kinetic energy (KE)of the projectile = .5*m*v^2 - an increase in diameter of 27% gives a mass increase of 1.27^3 (for the same shaped projectile), gives a KE increase of 1.27^3. So the 57mm projectile needs to have an increase in velocity of (1.27^3)^.5 to equal the KE of the 75mm projectile.

The metallurgy and machining of a high velocity gun barrel is more expensive than a low velocity barrel - which is why you see more in the way of low velocity 75mm than high velocity 57mm.

[BlackVoid]

AP projectiles contain no explosives, for these there are 2 basic factors.

- Kinetic energy - this depends on speed and mass of the projectile. The speed depends mainly on the length of the gun (also quality), mass is mainly a function of caliber. Barrel length is indicated generally as the multiplication factor of the caliber.

- Hardness - this is basically the quality of the ammo, the harder the projectile, the better the chance it will penetrate. Low quality armor shatters easier. This is one reason why most Soviet guns underperform.

Tungsten shot is harder and weighs more = increased penetration.

HC shot in the game is a charged projectile, it explodes on impact. Speed in this case is irrelevant and therefore the ammo has the same penetration values at all distances - at least in the game this is so.

The Soviet 57mm AT gun is very long (L73), thus it has very high speed. That is why it is much better than the standard 76mm AT gun.

Knowing what kills what and how (side/front, etc) is a daunting task. Stick to playing one narrow time period until you know the relative strength and weaknesses.

Detailed unit databases:

http://users.erols.com/chare/cm/

In the above tables with newer Excel versions you need to select all cells and set character color to black, otherwise they will look empty. Very useful at work where you do not have access to CM, but noone will complain if you are studying some large Excel tables.

[Michael Emrys]

AP projectiles contain no explosives...

Not always true I think. The Germans allegedly put small explosive charges in some of their rounds set to go off after the armor had been penetrated. I don't know if other nationalities practiced the same technique.

Michael

[John Kettler]

BlackVoid,

AP projectiles which kill by kinetic energy are broadly divided into two classes: AP shot, which is solid and has no explosive bursting charge, and AP shell, which does have a bursting charge. The British 2 pdr fired AP shot, whereas the German 3.7 cm Pak 36 fired AP shell, as did it's virtual copy the U.S. M6 37mm antitank gun. What the game calls "T" is a form of AP shot, consisting of a dense tungsten penetrator clad in a permanently attached light alloy sleeve. For the Germans, PzGr39 is AP shell, PzGr40 is AP shot. The Russians used AP shell, unless the target was very tough, in which case their "T" ammo, arrowhead, was used. The American 75mm gun on the Sherman used AP shell, but the British didn't trust the fuzes and plugged them, turning AP shell into AP shot. Also, they were loath to give up any potential penetration at all. The American "T" ammo was called HVAP, for Hyper Velocity Armor Piercing. The British called "T" Armour Piercing Composite Rigid, more commonly Hyper Shot and pioneered APDS, Armor Piercing Discarding Sabot, which is like "T" except that the light cladding falls away as soon as the shot clears the muzzle. This not only provides the penetration punch of "T" via high velocity, high density of the penetrator and small impact area, but now greatly reduces drag and enhances range. Another way to get a similar result was the squeezebore method in which the barrel was tapered and the projectile had light alloy flanges which compressed as the projectile screamed toward the muzzle. The Germans had this in such Gerlich guns as the 28/20, and the British came up with a device called the Littlejohn adapter to get a similar net result for armored car 2 pdr cannon.

I'd hoped to include a really great set of cutaway drawings, but I can't find them anymore on The Russian Battlefield. So here are a few from elsewhere to illustrate the concepts.

PzGr 39 from a Tiger 1. Note the cavity for the burster charge.

http://www.fprado.com/armorsite/Tiger1-2002-Picz/Tiger1-Ammo.jpg

By contrast, here's a PzGr 40 projectile for a 5 cm Pak 38. The dark area is the tungsten penetrator, and the silver part is light alloy, which impact strips from the penetrator.

http://sus3041.web.infoseek.co.jp/contents/shell_db/50_pzgr40_apcr.htm

Here is an overview of various types of AP shot. The dartlike affair bottom right is what modern tanks use.

http://homepages.solis.co.uk/~autogun/APtypes.jpg

Same thing, but for German ammo.

http://www.lonesentry.com/articles/ttt08/pics/armor-piercing-projectiles.jpg

Regards,

John Kettler

[Joachim]

The game gives the armor thickness and penetration in colors. Armor is displayed in the unit icon in the panel for turret, upper and lower hull / front, sides and rear each. Plus the top thickness.

Penetration is marked in the unit info (shown after hitting return/enter).

Color ratings:

Excellent = Blue, dark green, light green, yellow, orange, red = bad

green pen hits red armor = sure penetration (except at unfavorable angles)

red hits green = penetration once in a blue moon.

The big advantage of "thick" projectiles is the "behind armor effect". Partially due to sheer weight, partially due to HE load in larger warheads. A penetrating 76mm (large HE load) from a T34 will usually devastate the interior of a PzIII - while the Tungsten 50mm of the PzIIIj might penetrate the T34 but not disable the tank.

[John Kettler]

Joachim,

There's more to the AP shell story than meets the eye. You see, operational research in the Western Desert found that if AP shell penetrated even 50% of its length into the fighting compartment and detonated, the crew was generally rendered nonfunctional and the tank destroyed beyond repair, since the tank burned, destroying the protective properties of the armor steel.

Please see these threads, particularly my comments and replies thereto.

http://www.battlefront.com/community/showthread.php?t=42330&highlight=operational+research

My long post here on page three generated quite a bit of useful discussion.

http://www.battlefront.com/community/showthread.php?t=52117&highlight=operations+research

I fear a board crash wiped out some of the key data on these boards, but on Tanknet, our own Jeff Duquette discusses the very Beda Fomm tests I was looking for, together with some eye popping later ones.

http://208.84.116.223/forums/lofiversion/index.php/t19833.html

Regards,

John Kettler

[Joachim]

No RL data here - just CM experience put into a simple rule for CM:

The bigger the gun relative to the armor the better the chance of a kill given penetration or partial penetration.

Read: A Tiger I with its 88mm gun has a better kill chance than a Panther though the penetration probability is comparable.

[John Kettler]

Joachim,

The Panther has better penetration(176mm at 100 m, vs. 154, per CMBB Strategy guide, pp 2/44,2/54) than does the Tiger 1, but the Tiger has more explosive in the burster charge, hence, greater lethality.

Regards,

John Kettler

[John Kettler]

Joachim,

There's also the additional volume of armor displaced by the 88, which may be offset to some degree by the higher striking velocity of the Panther's very long 75.

Regards,

John Kettler

[Deadmeat1471]

Not at all a silly question, noxnoctum. Not a trivial question either.

Let's see... umm. Mostly you've got the answer - the higher velocity of the 57mm gun gives better penetration than a lower velocity 75mm gun. But a 75mm projectile with the same velocity as the 57mm will have better penetration (for a projectile made of a given material):

By this, i am thinking that the 57mm will be more accurate and can fire longer range than a 75mm due to the lighter weight, but the 75mm can pack more of a punch at a closer range?

Surely the 75mm is placed in a more powerful gun, thus giving it more kinetic force to counterbalance the increased weight?

my silly question.

[Michael Emrys]

By this, i am thinking that the 57mm will be more accurate and can fire longer range than a 75mm due to the lighter weight, but the 75mm can pack more of a punch at a closer range?

Er, it's a bit more complicated than that. It all depends on which 57mm and which 75mm we are trying to compare. I think we started out with the short barrel (24/cal) 75, which is a low velocity gun. But as the Pz. IV got a longer (first 42/cal then 48/cal) gun it fired a higher and higher velocity round, ending up with better penetration than the 57mm.

The other thing you need to understand is that if you take a 57mm and a 75mm gun (just for examples) and fire shots or shells of the same density from them at the same muzzle velocity, the 75mm will retain more of that velocity downrange for a given ballistic shape than the 57mm. That's because the frontal area of the shell increases only as the square of its bore, but the volume, and thus the weight, increases as the cube.

That's one of the things that made the German 88 such a fearsome long range tank killer. It still had a lot of its muzzle velocity even after it had traveled 1000 meters.

Michael

[Deadmeat1471]

sorry to have dumbed down a complex issue i was aware even if correct, my statement was grossly simplified, apologies.

Do you know of anywhere i could read about this subject? specifically the very things we are talking about, artillery pieces and armor penetration?

As you are aware, my knowledge on the subject is.... limited.

Question -

IF a 42 cal gun is generically 'better' than a 24 cal short gun, then why use short barrels at all? (i have often wondered this in ignorance), is it the weight of the artillery piece?

and the 88/L56 trumps both due to increased weight AND velocity.

Saying that , a pak 42 has

# Caliber: 7.5 cm

# Barrel length in calibers: 70, the velocity of this gun is 20% or so higher than the 88, with a 10% lesser round? seems a superior weapon to me?

Edit* scratch that last Q, i understand now.

'That's one of the things that made the German 88 such a fearsome long range tank killer. It still had a lot of its muzzle velocity even after it had traveled 1000 meters.'

[costard]

IF a 42 cal gun is generically 'better' than a 24 cal short gun, then why use short barrels at all? (i have often wondered this in ignorance), is it the weight of the artillery piece?

Hi Deadmeat. Better for knocking out armour, but short barrels have their advantages: cheaper to produce and can be manufactured in a relatively basic machine shop, a ballistic arc to place shells behind or over terrain, smaller propellant charge so more ammo in your vehicle. And weight, of course, as you noted, so you can use a lighter (i.e. more mobile) vehicle.

Saying that , a pak 42 has

# Caliber: 7.5 cm

# Barrel length in calibers: 70, the velocity of this gun is 20% or so higher than the 88, with a 10% lesser round? seems a superior weapon to me?

Ok, do the sums: KE(75/70)= 0.5*(m*0.9^3)*v*1.2^2, roughly, at the muzzle. Equals 1.05 times the KE of the 88/56 projectile.

[costard]

Because we've taken into account the variance in mass in the KE equation, we can say that only the cross-sectional area of the shell matters as far as the loss due to air resistance is concerned. The bigger shell will encounter more air, and lose velocity quicker. So the 88/56 will not outrange the 75/70 for KE.

Where the 88 comes up trumps is in the size of the bang it delivers: 1.33 (1.1^3) times the bang of the 75 (because of the increased volume of HE in the shell).

[Cuirassier]

"Hi Deadmeat. Better for knocking out armour, but short barrels have their advantages: cheaper to produce and can be manufactured in a relatively basic machine shop, a ballistic arc to place shells behind or over terrain, smaller propellant charge so more ammo in your vehicle. And weight, of course, as you noted, so you can use a lighter (i.e. more mobile) vehicle."

Add to that longer barrel life. And the shells could use a thinner shell casing, allowing one to fit more HE in there. Thus, short barreled gun were used initially has HE chuckers. Armies switched to higher-velocity, dual purpose guns later because multi-role function of tanks was important and vehicles often did not last long enough to wear out the barrel anyway.

[Michael Emrys]

The bigger shell will encounter more air...

Yes, in proportion to the square of its cross section.

...and lose velocity quicker.

No. It retains velocity longer, especially if it started out a bit slower. Air drag is determined by several things. One is the frontal area. One is the aerodynamic profile. But also very important is velocity. Drag goes up as the square of the velocity (although funny things can happen around the speed of sound). So if a shot starts out slower, is is subject to less drag.

But even if they start out at the same velocity at the muzzle, the larger projectile will lose velocity more slowly. Why? Because it has greater mass in proportion to its frontal area. And mass means inertia, and it's inertia that is fighting drag.

Michael

[costard]

Deadmeat - Marines Ballistics Manual here.

[Deadmeat1471]

Thanks for the detailed answers, Im a great enthusiast of history, but some of us are still ballistics newbies

[costard]

And some of us are still learning, even though we think we know it all. Michael, see your sig.

[Col Deadmarsh]

...kinetic energy (KE)of the projectile = .5*m*v^2 - an increase in diameter of 27% gives a mass increase of 1.27^3 (for the same shaped projectile), gives a KE increase of 1.27^3. So the 57mm projectile needs to have an increase in velocity of (1.27^3)^.5 to equal the KE of the 75mm projectile.....

*reaches for the aspirin*