This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.
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The Sherman tanks with the 76mm gun were known to have a less effective HE round than their predecessors with the 75mm gun. I have heard various excuses as to why, but most imply that there was some reason that only high-velocity rounds could be used and this required thicker shell walls and less volume of HE filler. Was there some reason why a lower-velocity shell with a larger HE fill was not developed? It appears that the 76mm gun had a significantly slower twist rate than the 75mm. Was this the whole reason, or was there another?
I had originally thought that the whole purpose was to insure that the HE round matched the trajectory of the original AP one to simplify the sighting reticle and reduce confusion. Then I realized that the flatter-shooting HVAP-T would have not matched the marks on the reticle either, yet the crews adapted.
Bonus Question!: What was the reason that the twist rate of the 76mm gun was increased from 1-in-40 calibers to 1-in-32 calibers? This compares to the 75mm's twist of 1-in-25 calibers, according to one reference, FWIW.
The standard HE shell for the various US 3 inch/76mm guns in WW2 was the M42A1, which weighed 12.87 lb and was normally filled with 0.86 lb of TNT.
This shell was used with three different sizes of cartridge case:
- 76.2 x 583 x 108 rim (3 inch Mk II M2 case for 2,800 fps) for all 3 inch / 76 mm seacoast /AA / AFV guns except those listed below:
- 76.2 x 675 x 119 rim (3 inch Mk I M2 case for M1903 seacoast gun only;
- 76.2 x 539 x 90 rim (M26 case, 3.62 lb propellant for 2,700 fps) for 76mm tank guns only.
Maximum chamber pressure for all was 36,000 psi, except for the 76mm tank gun; 43,000 psi.
By comparison with the US 75 mm tank gun: standard HE shell was the M48, weighing 14.7 lb with 1.47 lb TNT.
So the 75mm shell had a 10% filler weight, the 76mm only 6.7%.
Incidentally, the British 17 pdr HE shell was initially no better than the US 76mm, but later Marks saw a reduction in velocity.
Was there a technical reason why the 76mm tank gun could not use a shell with more HE filler? It seems that the lower HE charge was commonly cited as a major disadvantage of the new gun versus the 75mm. As you noted, the 17 pdr eventually got improved HE shells. I wonder why the 76mm did not also get an upgrade, especially after the twist rate change.
Iirc, they were mostly concerned about the effectiveness in the AT role, so they decided the HE round was "good enough".
The shell in question, the 3'' / 76 mm M42A1, was simply old design, of WW1 era (M42 was slight modification of Mk. IX, which itself apparently was shortened Mk. I, introduced with M1917 AA gun; see: https://bulletpicker.com/pdf/TR-1360-3A-1928.pdf p. 6 of the file and https://bulletpicker.com/pdf/TR-1360-3A-1936.pdf p. 16). Shells of that era had cylindrical HE cavity, i.e. with constant wall thickness.
More modern had varying wall thickness and could both withstand high acceleration in bore and contain more explosive.
Post-WW2 76,2 mm HE ammunition for guns used in M41 light tank had MV of 2400 ft/sec and those shells contained about as much explosive as WW2 era 75 mm M48 shell.
I guess it was low on the priority list during WW2 to spend resources on better HE ammunition for 3'' M7 and 76 mm M1 guns, which were intended primarily as armor killers.
Thanks for that. What had me confused was that I read that the 75mm production had been completely stopped, so, at some point, the 76mm guns would be used mainly used in the same infantry support role as its predecessor. It seemed logical that a comparable HE round would be needed. I am aware that the cannon was designed to use the same projectiles as the 3", but my understanding is that even the tank destroyers using 3" ammo shot a lot of HE and would have preferred a more effective round.
Maybe the sheer amount of surplus ammo from WWII production led to the "it's good enough" decision.
The earlier 76mm guns like the M1A2 in the Sherman stayed around for a long time, although largely used by other countries. Do you know if any improved HE round were developed for or by them? Could the M352 used by the M41 tank be fired by the older cannons?
Perhaps having the AP and HE rounds have the same velocity was a factor, making aiming simpler.
That might be the case, but the HVAP round that appeared in late 1944 had a different velocity than the previous AP and HE, so I doubt that wanting to match velocity was the only issue. I am starting to lean towards the "it's already in production and it's good enough" school of thought.
As Kocur noted, the 76mm M32 gun used in the M41 tank did indeed have the M352 HE shell that had a lower velocity than the AP and HVAP and pretty much matched the old 75mm shell's explosive content. This indicates a change of heart for one reason or another after WWII.
Still a mystery, I guess. The Army did many silly things in the war, like not training the troops in North Africa how to use a bazooka before the were deployed to the front and not teaching tankers how to use the gyro-stabilizer, which was very effective for trained crews and not at all for the untrained. They also cut a foot off the ww2 76mm gun instead of adding weight to the back of the M18's turret.
I can't see any mystery. M42A1 was US Army standard 3'' HE shell (apart from tank guns it was used in AA guns and seacoast guns) and the only one in production.
That it was substantially less effective than 75 mm M48 against ground target was really noticed only after Normandy, when 3''/76 mm guns in GMCs and tanks saw wider use. There was not enough time to replace it before end of WW2.
Even if it was noticed earlier, better shell would have faced a lot of competition from other weapons programs.
Apart from manufacturing issues, often cited reason to develop US first electronic computer was to speed up calculations for firing tables for all the new ammunition being developed during WW2. I mean, that too apparently was a bottleneck.