U.S. 40mm M81 AP Shot

[rexford]

Does anyone have a drawing of the subject rounds (M81 and M81A1) that would show if they had a flat or blunt nose.

A friend who was stationed at Fort Drum in Watertown NY told me that anti-aircraft rounds would sometimes have flat nose, which were designed to be extra effective against thin skin targets. While a pointed nose might blow through the armor, in one side and out the other, a flat nose round would use lots of energy getting though armor and take a big chunk of plate with it, increasing chance of major damage and injuries.

Military folks referred to flat nose anti-aircraft rounds as "wad cutters" because they blew fairly large pieces of armor into the aircraft instead of cutting nice neat holes with petal shaped fold back edges.

Flat nose rounds also limited the range of the projectile, which could prevent practice rounds going off a base and striking houses and cars.

Thanks for any help that can be provided.

[rexford]

Penetration data for 40mm M81 AP suggests that it might be flat nose. 2 pounder AP penetrates about 87mm of homogeneous armor at 2600 fps and 2.375 pound weight, 40mm M81 AP penetrates 61mm at same velocity and 1.96 pounds. Both rounds are solid AP shot without caps.

When U.S. 40mm M81 AP penetration is adjusted upward for lower weight, penetration at 2600 fps and 2.375 pounds would be 70mm, still alot less than 2 pounder AP.

So U.S. 40mm AP M81 is either much lower nose hardness than 2 pounder, or 40mm is a flat nose projectile.

[Jeff Duquette]

26. Cartridge, 40 Millimeter: AP-T, M81

The cartridge M81 differs from the cartridge M81A1 (par. 27) only in the method of attaching the windshield and the primer used. This cartridge has a windshield that is secured to the projectile by means of an adapter; the cartridge M81A1 has a windshield that is crimped directly to the projectile. This cartridge is assembled with the percussion primer M23A2; the cartridge M81A1 uses the percussion primer M38A1. Some of these cartridges, intended for use during the early stages of training, have a vapor-trail-producing dipcoating compound on their windshields.

=====================================

27. Cartridge. 40 Millimeter: AP-T, M81A1

a. General. This cartridge is used for firing against armored and similar targets. The hardened steel projectile is of the monobloc type; a tracer cavity in its base contains a red tracer composition which burns for about 12 seconds. The nose of the body proper is shaped to a relatively blunt ogive. However, a long false ogive is provided to impart optimum ballistic properties, by means of a lightweight windshield (ballistic cap) secured to the projectile body. The windshield is attached to the nose crimping groove by means of a 360-degree crimp. Certain lots of these cartridges having defective tracer elements have a vapor-trail-producing dipcoating compound applied to their windshields. The dipcoating compound will produce a vapor trail that begins immediately after the projectile leaves the muzzle of the gun and continues to be visible up to a range of 1,000 yards. Visiblility of the vapor trail is limited to daytime firing. Ammunition having this dipcoating is intended for use during the early stages of training only and is not intended for use in the final stages of training or in combat except in an emergency. This cartridge is also issued with a steel case.

b. Data.

Weight of cartridge ..............4.58 lb

Weight of projectile, as fired ...1.96 lb

Length of cartridge .............17.60 in.

Length of projectile ............ 6.l9 in.

Length of cartridge case ........12.24 in.

Width of rotating band ...........0.64 in.

Type of base ....................square

Radius of ogive .................5.96 cal.

Muzzle velocity ................2,870 fps

[ 07-08-2001: Message edited by: Jeff Duquette ]

[Stacheldraht]

You can find official data and diagrams on these and other rounds here: http://carlisle-www.army.mil/usamhi/DL/chron.htm#AWorldWarII19391945

Volume III, p. 509 of the Standard Ordnance Items Catalog.

[rexford]

Never saw that Carlisle site before, great info! Thanks.

40mm M81 AP is for Bofors anti-aircraft gun, and has unusual nose and ogive shape.

This helps our research quite a bit.

[rexford]

Even though 40mm M81 AP doesn't have a flat nose there are similarities between that round and Russian APBC, which will be analyzed.

The 40mm AP data may help estimate Russian APBC penetration and slope effects against face-hardened armor.

[M Hofbauer]

I thought wad-cutter is a term that comes from civilian shooting and refers to the fact that a wad-cutter flat-nose projectile will stance a nice clean round hole out of a wad (or paper) as opposed to a regular projectile which will simply penetrate the target and make fuzzy holes (harder to determine exact score).

As for the 40mm Bofors ammo you might want to try old WW2 pictures of american AA emplacements. I remember I had borrowed a book once which showed a Bofors emplacement near the Remagen bridge and IIRC it showed ammo lying around for the AA gun. So historic pictures of Bofors guns, of which there should be plenty, might be of help.

btw, what's the status on your book?

[rexford]

Book is being sold now. Frederic Erk at Saumur Museum is handling European orders, I handle rest of world.

Book review is available at following site, go to articles, articles and essays and then halfway down page.

http://www.tabletoptactics.com/

For ordering info e-mail me at rexford179@cs.com or Frederic Erk at

curator@musee-des-blindes.intranets.com

Book is 136 pages text and about 64 other pages with graphs, drawings and tables.

Thanks for interest.

[rexford]

The data in TM9-1907 for 40mm M81 penetration leads to the following slope multipliers at 2250 fps impact:

Homogeneous Armor

1.15 at 30°

Face-hardened Armor

1.19 at 30°

1.37 at 45°

1.82 at 60°

AP rounds normally have slope effects over 1.25 for 30° and over 2.50 at 60° for the thicknesses penetrated by 40mm at 2250 fps. The above slope effects look like what would result from flat nosed rounds hitting sloped plate, rather than standard AP ammo.

Anti-aircraft rounds with flat noses called "wad cutters" cause they knocked out a wad of armor when they hit. Local term.

[Michael Dorosh]

<BLOCKQUOTE>quote:</font><HR>Originally posted by M Hofbauer:

I thought wad-cutter is a term that comes from civilian shooting and refers to the fact that a wad-cutter flat-nose projectile will stance a nice clean round hole out of a wad (or paper) as opposed to a regular projectile which will simply penetrate the target and make fuzzy holes (harder to determine exact score).

<HR></BLOCKQUOTE>

I remember selling a lot of SWC at the gun shop; IIRC it meant Semi Wad Cutter and was used by pistol shooters in the method you describe.

Could I be any more irrelevant?

[Lee]

rexford: Have you presented your findings to Steve yet? I remember you

mentioning some very interesting information about CM likely having

too much penetration on German armor from certain types of AP rounds.

This was discussed at some length in a thread just before the forum crashed

a couple months ago. Unfortunately, I think most of that thread was lost

in the crash.

I'm very curious to see what Steve and Charles think of your findings

as it relates to what you brought up in that thread I mentioned above.

[rexford]

Sherman 75mm penetrates 81mm homogeneous armor at 500m and 95mm face-hardened.

Tiger uses all homogeneous armor

PzKpfw IVH is face-hardened on front

Panther A and D hull side is face-hardened

Tiger II curved 100mm mantlet is face-hardened

StuG III appears to be face-hardened on front

CM penetration data for Sherman 75 is inbetween TM9-1907 data against face-hardened and homogeneous, 89mm at 500m.

So CM may overestimate Sherman 75 against all-homogeneous armor Tiger and homogeneous Tiger II side, and underestimate effectiveness against face-hardened targets.

British tests of several different 75mm and 76.2mm rounds against their homogeneous armor resulted in the same homogeneous penetration for U.S. 75 and 76 as TM9-1907.

The 10% greater penetration for Sherman 75 in CM compared to TM9-1907 data means that Sherman pierces Tiger 80mm side at 500m with side angles from 0° to 21° in CM (half hits penetrate criteria). I've lost ALOT of Tigers within that angle spread.

If one goes with TM9-1907 data, Sherman 75mm at 500m has to hit Tiger 80mm side armor with side angles from 0° to 2° to penetrate on half the hits.

On the wargame battlefield, Sherman 75 gains a big advantage against Tiger side armor if penetration is 89mm at 500m, compared to results with 81mm penetration at 500m. That 10% difference means a big change in the penetration angles (a much wider arc for successful hits).

Our book analyzes Allied firing test data against Tiger and Panther and shows that German armor quality was the same as Allied penetration test plate. So U.S. and British penetration data can be directly compared to German armor.

Not to appear too Tiger oriented, but our book also uses 82mm for Tiger side armor instead of 80mm. Every little bit helps, ya know.

82mm for Tiger side armor is only 2.5% more thickness, but if that is what the British measured, that is what they measured. Russians seem to use 82mm quite a bit too in their analyses.

To be fair, our book presents actual measured thickness for 76mm armed Sherman mantlet, which is ALOT THICKER than the 89mm figure that is normally assumed.

Russian armor is usually thinner than spec thickness when it is measured, but only for certain vehicles.

[ 07-09-2001: Message edited by: rexford ]

[M Hofbauer]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Michael Dorosh:

Could I be any more irrelevant?<HR></BLOCKQUOTE>

*g*

Mike,

yes your contribution might be even more useless then my suggestion to "look at pictures"

semi-wadcutter is more common than pure WC; pure wad cutters have such a poor aerodynamic performance that they are avoided; but they are still quite numerous here among shotgun ammunitions, because solid-projectile shooting at targets from shotguns isn't exactly sniping anyhow, if you know what I mean...

I think there might be more money on my theory, because I think the term wadcutter is quite a bit older than the use of metal-skinned aircraft.

[Jeff Duquette]

I think the wad-cutter reference is purely an analogy to the “flatter” looking penetrator of the M81 and M81A1. Ultimately I believe what Rexford is driving at is the contrasts between Soviet built APBC penetrators and more pointy penetrators employed by western types in their APCBC rounds. Shape effects of penetrator on armor perforation.

The M81 is being employed as an example as it is sort of flat and ballistic test data is readily available.

Better would be Soviet Test firing reports of APBC vs FH armor. Equally important would be German test firing reports for captured Soviet equipment detailing ballistic testing. No rechambered 76.2mm testing using German manufactured APCBC, but honest to goodness testing of Soviet 76.2 using Soviet APBC ammunition against face hardened armor plate.

[Lee]

rexford: Those are very important findings for their implications in CM.

I certainly hope you will formally present them to Steve very soon,

so that he and Charles can look them over and make any necessary changes

to CM2's armor penetration modeling. I'm sure Steve will be eager to

see your data.

We all share a common goal of making CM as realistic as we can and

having accurate armor penetration is a very important part of that.

Thanks for doing such detailed research, it benefits all of us.

[Ari Maenpaa]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Lee:

rexford: Those are very important findings for their implications in CM.

I certainly hope you will formally present them to Steve very soon,

so that he and Charles can look them over and make any necessary changes

to CM2's armor penetration modeling. I'm sure Steve will be eager to

see your data.

We all share a common goal of making CM as realistic as we can and

having accurate armor penetration is a very important part of that.

Thanks for doing such detailed research, it benefits all of us. <HR></BLOCKQUOTE>

I very strongly agree with this statement.

Ari

[Username]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Michael Dorosh:

I remember selling a lot of SWC at the gun shop; IIRC it meant Semi Wad Cutter and was used by pistol shooters in the method you describe.

Could I be any more irrelevant?<HR></BLOCKQUOTE>

Did you sell alot of bait also? Would you clean anglers fishes' guts out and smile retardly too?

You probably worked at a place with a sign like...

"AMMO, BAIT, COLD CUTS and BEYOND..."

Lewis

[Lee]

Wadcutter ammo for handguns has a flat nose. sem-wadcutter has a

tapered shoulder to the bullet, making it more pointed, but it still

has a flat nose at the bullet tip.