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3-inch gun M7 vs 76mm M1A1


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shapeshifter #1 Posted Feb 19 2015 - 01:00

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Something that always seemed off to me.

 

3" gun M7

Length of bore 150 inches (50.0 calibers)

Rifling 28 grooves right hand twist one turn in 40 calibers.

Chamber capacity 205.585 cubic inches

 

76mm M1A1

Length of bore 156 inches (52.0 calibers)

Rifling 28 grooves right hand twist one turn in 40 calibers.

Chamber capacity 142.6 cubic inches

 

Shell case size

 

 

3" gun M7

 

HE M42A1


Cartridge case: Mk. II M2 

Chamber cap.: 203.50 cubic inches

Proj. weight: 12.87 lb
Complete weight: 24.91 lb
Proj charge: 0.86 lbs TNT bursting charge or 50/50 amatol and cast TNT booster surround.
Propelling charge: 4.87 lbs M1 class powder 87-10-3 formula
Velocity: 2,800 f/s
4.27 " dia
34.05" long
 

 

APC M62A1


Cartridge case: Mk. II M2 

Chamber cap.: 205.585 cubic inches

Proj. weight: 15.43 lb
Complete weight: 27.23 lb
Proj charge: Explosive "D" 0.144 lb
Propelling charge: 4.87 lbs M1 class powder 87-10-3 formula
Velocity: 2,600 f/s
4.27 " dia
35.54" long

 

 

76mm M1A1/etc

 

HE M42A1


Cartridge case: M26

Chamber cap.: 140.50 cubic inches

Proj. weight: 12.87 lb
Complete weight: 22.11 lb
Proj charge: 0.86 lbs TNT bursting charge or 50/50 amatol and cast TNT booster surround.
Propelling charge: 3.75 lbs M1 class powder 87-10-3 formula
Velocity: 2,700 f/s
3.566 " dia
32.25" long
 

 

APC M62A1


Cartridge case: M26

Chamber cap.: 142.6 cubic inches

Proj. weight: 15.43 lb
Complete weight: 24.55 lb
Proj charge: Explosive "D" 0.144 lb
Propelling charge: 3.75 lbs M1 class powder 87-10-3 formula
Velocity: 2,600 f/s
3.566 " dia
33.80" long

 

3" gun rounds have much more propellent, and this shows with the HE round having a higher muzzle velocity. yet the AP rounds all have the exact same specs and velocity as on the 76mm model guns. the 3" M62A1 has 1.12 lbs of extra propellent then the 76mm cartridge.


But it's odd that most sources copy them carbon for performance but I have seen the M62A1 and M79 rounds listed as follows.

 

3" M62A1

20 deg @

500 yards 4.3"
1000 yards 3.9"


3" M79

20 deg @

500 yards 4.7"

1000 yards 3.9"

 

76mm M62A1

20 deg @
500 yards 4.3"
1000 yards 3.8"

 

76mm M79

20 deg @

500 yards 4.6"

1000 yards 4.0"

 

 

 

Another source

 

3" gun M7 M62A1

 

Face hardened armor

0 deg @ 1000 yards

4.7 inches

 

[edited]plate

0 deg @ 1000 yards

4.7 inches

 

3" gun M79

 

Face hardened armor

0 deg @ 1000 yards

3.1 inches

 

[edited]plate

0 deg @ 1000 yards

4.5 inches

 

 

76mm M1A1/2 gun M62A1

 

Face hardened armor

0 deg @ 1000 yards

4.7 inches

 

[edited]plate

0 deg @ 1000 yards

4.5 inches

 

76mm M1A1/2 gun M79

 

Face hardened armor

0 deg @ 1000 yards

3.0 inches

 

[edited]plate

0 deg @ 1000 yards

4.5 inches

 

Here's a thought experiment.

 

German L/43

German L/48

 

Pzgr 39


Cartridge case: 6339

4" dia at rim, 3.8" at shoulder, 3.15" at mouth
19.4" long

Chamber cap.: ? cubic inches

Proj. weight: 15.13 lb
Complete weight: 29.54 lb
Proj charge: 0.1825 lb Cyclonite
Propelling charge: 5.35 lb diglycol
Velocity: 2,460 f/s
 

 

 

German Pak 40 (L/46)

 

Pzgr 39


Cartridge case: 6340 st

3.937" dia at rim, 3.187" at shoulder, 3.062" at mouth
28.1" long

Chamber cap.: 217.885 cubic inches

Proj. weight: 15.13 lb
Complete weight: 33.42 lb ?
Proj charge: 0.1825 lb Cyclonite
Propelling charge: 5.975 lb diglycol ?
Velocity: 2,600 f/s
 

 

 

All three cannons used the same projectile, the L/43 and L/48 both used the same cartridge case with propellent. The Pak 40 used a longer thinner cartridge case that held more propellent overall and had increased performance over the L/43/48. (velocity and penetration)

 

L43: 2427 f/s
L48: 2460 f/s
L46: 2600 f/s

 

 

OX1BzmX.png


Edited by shapeshifter, Feb 21 2015 - 01:24.


Legiondude #2 Posted Feb 19 2015 - 01:16

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Didn't notice before, but you switched up the M62A1 with "M6A21"

shapeshifter #3 Posted Feb 19 2015 - 01:19

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View PostLegiondude, on Feb 18 2015 - 19:16, said:

Didn't notice before, but you switched up the M62A1 with "M6A21"

Opps, corrected.



madgiecool #4 Posted Feb 19 2015 - 01:21

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Nice post!  Grats on you time and research.

Anlushac11 #5 Posted Feb 19 2015 - 02:54

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Here's a Interesting story.

 

After Brits recaptured Tobruk they captured large stocks of 75mm PzGr.39 rounds intended for the new PzIV-F2's but were unable to be delivered due to complete aerial dominance by the Allies.

 

A Capt. George B. Jarrett got the idea to compare the captured German 75mm PzGr39 rounds and compared them to US  AP rounds.

 

IIRC there was a problem with some US 75mm and 76mm AP rounds (M61? M62?) due to poor quality control and improper heat treating at certain plants. The Brits made use of the captured ammo by adapting the German 75mm APCBC rounds to fire from US 75mm guns.

 

It was found that with slight modification and machining the PzGr 39 projectile would fit a US 75mm shell casing. The Brits converted a large number of these captured rounds. The explosive charge in the projectiles were removed so there would not be any post penetration effects.

 

Block Quote

 Adapting captured 75-mm. APCBC (armor-piercing-capped, ballistic-capped) ammunition for use in the American Grant tank's 75-mm. gun, which meant reworking the rotating bands, was a major effort in the Royal Army Ordnance Corps workshops in preparing for the May 1942 offensive, an effort to which Major Jarrett contributed so largely that he was decorated by the British Government.

 

 

 


Edited by Anlushac11, Feb 19 2015 - 03:00.


Dominatus #6 Posted Feb 19 2015 - 03:08

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As per Hunnicutt, the penetration was unaffected, only damage to the target increased.

shapeshifter #7 Posted Feb 21 2015 - 02:33

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Some interesting info. When they were testing the T1E2 they tested the gun as well. 3" AA gun T12.

Gun was designed for recoil of 12 to 14 inches.

 

First test is just listed as  "15 lb projectile" Second test had 3" M1? or VII inert training rounds scan is blurry (can't find any M1 training round but there is a VII round from the 3-inch M1903 coast gun) weighed 15 lbs. Third test was with Mark 1 HE shells that weighed 15 lbs.

 

Shell weight: 15 lb

MK II case: 6.66 lb

Propellent : assuming 4.87 lb standard.

Total: 26.53 lb

 

The M7's HE shell the M42A1 weighed 12.7 lbs with 2800 f/s and it's solid AP shot the M79 weighed 15 lbs yet was listed at 2600 f/s.

 

Test #1 (Jan? 1942)

 

22 rounds total fired.

increasing from 50% normal pressure to 165% last few rounds were fired from 99% to 108% normal velocities.

 

Round #11 165% pressure

Round #22 108% velocity

 

Rounds fired at "normal" velocity had recoil measured at 9-7/8 inches to 10-1/4 inches with incomplete ejection.

 

Results. Gun had case ejection issues with them only partially coming out. Suggestions made for remedy.

 

In test #2 (Feb? 1942) (using 90mm propellent powder type)

 

100% charge gave 2600 f/s with 10-1/2" recoil

110% charge gave 2700 f/s with 10-3/4" recoil

115% charge gave 2800 f/s with 11-1/4" recoil

 

Results. Gun still has case ejection issues with them only partially coming out.

 

In test #3 (April 1942) (using 90mm propellent powder type)

 

They don't list the charge amounts but list two rounds as fired at 2600 f/s one at 2700 f/s and the rest (majority) fired at 2800 f/s

 

Results. List no problems or malfunctions encountered in this test.

 

sort of reads like the ejection issues were caused by the "normal" velocity rounds (2600 f/s) and upping the charge did the trick to eject the shells and getting the recoil to the rated numbers.

 

 


Edited by shapeshifter, Feb 21 2015 - 03:08.


shapeshifter #8 Posted Feb 21 2015 - 06:42

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View PostAnlushac11, on Feb 18 2015 - 20:54, said:

Here's a Interesting story.

 

After Brits recaptured Tobruk they captured large stocks of 75mm PzGr.39 rounds intended for the new PzIV-F2's but were unable to be delivered due to complete aerial dominance by the Allies.

 

A Capt. George B. Jarrett got the idea to compare the captured German 75mm PzGr39 rounds and compared them to US  AP rounds.

 

IIRC there was a problem with some US 75mm and 76mm AP rounds (M61? M62?) due to poor quality control and improper heat treating at certain plants. The Brits made use of the captured ammo by adapting the German 75mm APCBC rounds to fire from US 75mm guns.

 

It was found that with slight modification and machining the PzGr 39 projectile would fit a US 75mm shell casing. The Brits converted a large number of these captured rounds. The explosive charge in the projectiles were removed so there would not be any post penetration effects.

 

Block Quote

 Adapting captured 75-mm. APCBC (armor-piercing-capped, ballistic-capped) ammunition for use in the American Grant tank's 75-mm. gun, which meant reworking the rotating bands, was a major effort in the Royal Army Ordnance Corps workshops in preparing for the May 1942 offensive, an effort to which Major Jarrett contributed so largely that he was decorated by the British Government.

 

 

 

Seems they tested the L/24 rounds in Cairo.

 

 

 

 

Americans tested this as well at Aberdeen, but with the rounds used in the L/24.

 

 

Chances are the AP round in that test was the K.Gr.rot.Pz.

which was very different from the pzgr 39. Much larger bursting charge in the K. Gr, but the 39 had much better armor penetration capabilities.

 


Edited by shapeshifter, Feb 21 2015 - 07:11.


Anlushac11 #9 Posted Feb 21 2015 - 11:54

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View Postshapeshifter, on Feb 20 2015 - 20:33, said:

Some interesting info. When they were testing the T1E2 they tested the gun as well. 3" AA gun T12.

Gun was designed for recoil of 12 to 14 inches.

 

First test is just listed as  "15 lb projectile" Second test had 3" M1? or VII inert training rounds scan is blurry (can't find any M1 training round but there is a VII round from the 3-inch M1903 coast gun) weighed 15 lbs. Third test was with Mark 1 HE shells that weighed 15 lbs.

 

Shell weight: 15 lb

MK II case: 6.66 lb

Propellent : assuming 4.87 lb standard.

Total: 26.53 lb

 

The M7's HE shell the M42A1 weighed 12.7 lbs with 2800 f/s and it's solid AP shot the M79 weighed 15 lbs yet was listed at 2600 f/s.

 

Test #1 (Jan? 1942)

 

22 rounds total fired.

increasing from 50% normal pressure to 165% last few rounds were fired from 99% to 108% normal velocities.

 

Round #11 165% pressure

Round #22 108% velocity

 

Rounds fired at "normal" velocity had recoil measured at 9-7/8 inches to 10-1/4 inches with incomplete ejection.

 

Results. Gun had case ejection issues with them only partially coming out. Suggestions made for remedy.

 

In test #2 (Feb? 1942) (using 90mm propellent powder type)

 

100% charge gave 2600 f/s with 10-1/2" recoil

110% charge gave 2700 f/s with 10-3/4" recoil

115% charge gave 2800 f/s with 11-1/4" recoil

 

Results. Gun still has case ejection issues with them only partially coming out.

 

In test #3 (April 1942) (using 90mm propellent powder type)

 

They don't list the charge amounts but list two rounds as fired at 2600 f/s one at 2700 f/s and the rest (majority) fired at 2800 f/s

 

Results. List no problems or malfunctions encountered in this test.

 

sort of reads like the ejection issues were caused by the "normal" velocity rounds (2600 f/s) and upping the charge did the trick to eject the shells and getting the recoil to the rated numbers.

 

 

 

Hmmm that muzzle is way too high for the HE round. 2800fps would be very close to the 76.2mm M62A1 APC-T round.

 

I would expect HE round muzzle velocity should be about 1500fps-1700fps.



shapeshifter #10 Posted Feb 21 2015 - 21:01

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Why would it be that low? as I pointed out the 3 Inch gun used much more propellent then the 76mm gun.



Anlushac11 #11 Posted Feb 22 2015 - 18:25

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Either gun would have a lower velocity HE round,

 

Since 3"M7 and 76mm M1 shared same projectiles but used a different casing they probably used same projectile. IIRC HE projectile for both is M42A1.

 

In WW2 it was found that HE rounds fired from high velocity guns like German 75mm L/70 KwK 42 and OQF 17lbr had two likely bad outcomes.

 

1) Round would hit and either bounce or the round would be destroyed on impact and fail to detonate.

 

2) The round would penetrate "softer" targets like buildings and explode on backside of building, making a nice entry point for enemy troops but not helpful in creating a entry point for your troops to enter a building.

 

IIRC the solution for both weapons was to reduce propellant loading which reduced chamber pressure and thus muzzle velocity.

 

As for the M62A1 fired from 3" and M1A1 I dont have a answer for you. The 3" I looked up does show better muzzle velocity at 2800fps while M62A1 fired from 76.2mm M1A1 is listed at 2600fps.

 

While the 3" M7 has more powder it also has a 50 caliber barrel while M1A1 has a 52 caliber barrel, the longer barrel would allow more time for more energy to be transferred to the projectile but the difference I would not expect to be as high as 200fps.

 

German 75mm L/43 and L/48 IIRC used same rounds with the L/48 having a longer barrel but muzzle velocity difference mentioned was about 32fps.

 

EDIT: Did some digging and looks like with both weapons firing the same PGr39 APCBC round the German 75mm L/48 KwK 39 had 164 fps more muzzle velocity

 

I am now more inclined to think that yes M7 had 1lb more propellant for AP round but the M1A1's longer barrel helped it achieve comparable velocities. So Im guessing less propellant used equals lower chamber pressure and longer barrel life over the M7. Also by using less propellant for same projectile the propellants produced goes farther which helps war effort.

 

 


Edited by Anlushac11, Feb 22 2015 - 18:51.


shapeshifter #12 Posted Feb 22 2015 - 21:47

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They had a reduced charge HE shell for both guns. But the standard was the full propellent.

shapeshifter #13 Posted May 03 2015 - 04:14

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From Ian Hoggs work.

 

Would be interesting to look at development reports on the M5, perhaps they had a bigger charge for use on the carriage.


Edited by shapeshifter, May 04 2015 - 18:05.


shapeshifter #14 Posted Aug 10 2016 - 06:12

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Canadians talking about the T12 3 inch gun (the M7) in 1942

 


"The 3-In. T12 gun, which is virtually the remodeled U.S. 3-in A.A. gun mounted in this way, is the most desirable weapon yet produced for front line artillery work with tanks. It is a tank buster in the true sense of the word, and it will, in this writer's opinion, be capable of taking on anything in the way of a tank that the enemy can produce. It's present muzzle velocity is stated as 2650 f/s/. but it has been intimated that the development of a super-charge will give performance close to that of the British 17-pr., which gives over 3000 f.s. This gun has a sufficient safety factor to be able to use super-charge ammunition, while the 76mm - the new 3" which is considered for the M4, having a safety factor of approximately 1.5? could not.

 

The U.S. Ordnance, when designing this buffer and recuperator, had in mind the possibility that it might be used with the British 17-pdr. There is still this possibility, and we are informed that the British production of the 17-pr. barrels is such that some might be diverted for use on this mount. We have not as yet been advised that the British have developed a satisfactory buffer and recuperator of their own for the use of a 17-pr. in a turret. In any event there appears to be sufficient 3-in. barrels available in the U.S. and, if necessary, the 76-mm is still available and can be used."

 

 

We know the 3 inch can hold quite a bit more powder, but top velocity is a ? here's what the 76mm M1A2 could do with better powder.

 

 

 

General Barnes had stated that the 3-inch fired at 2800 ft/sec as well in a book of his.

 

 

The fact they state that the buffer/recuperator would work with the 17-pdr, I wonder if that has to do with early on in the T1/M6 development the British asked/suggested the 17-pdr to be mounted in it.


Edited by shapeshifter, Aug 12 2016 - 20:27.





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