When the Army realized that the M2 and M3 guns in the M3 and M4 tanks weren't going to cut it for most anti-tank purposes...
OK, they didn't. The M2 and M3 guns do exactly what they were supposed to do, and the Gun Motor Carriages were supposed to handle the stuff that was too heavy for the tanks.
So they made what we call "tank destroyers," or gun motor carriages.
The TD counterpart to the Sherman is the Gun Motor Carriage M10.
It's got a long 3" gun that's tailored to kill tanks. This is the M7 gun on an M5 mount.
Towards the end of WW2, they started to replace the M10 with the Gun Motor Carriage M18 "Hellcat". It too has a long 3" gun that's tailored to kill tanks. This is the M1 series of guns.
The tank destroyer concept started to wane after the war and we developed a light tank with a long, 3" gun that had a decent chance against a tank. This is the M32.
They all use the same round, right?
No.
No, they do not.
Despite all sharing the same caliber, they don't share the same case.
Frustratingly, The M7 and M1 guns share projectiles. The cartridges are named after the projectile so they share designations. 76mm HE M42 for the M7 is a different round from the 76mm HE M42 for the M1.
The M7 gun fires 76.2x585mmR.
The M1 series fires 76.2x539mmR.
The M32 fires 76,2x580mmR.
The manuals that ordnance issues doesn't really distinguish between the 76mm rounds and even forgets there's more than one cartridge length.
What caused me to notice was going through the rounds for the Lee/Grant and Sherman's 75mm M2 and M3 guns. Those guns both fire 75x350mmR.
But Ordnance doesn't specify that the 75mm Pack-Howitzer rounds are not the same.
The round that started my noticing was the 75mm M66 HEAT-T round and seeing how low the muzzle velocity was compared to other rounds fired by the Sherman. Even lower than the shorter barreled gun in the Lee/Grant.
This is because the pack-hog fires 75x272mmR! It gets a couple of rounds that the Sherman never got because it stayed in service a lot longer.
It's getting sorted.
I have also made my first set of stats for a NBC round. The M64 Chem round. Mustard gas! Not to be confused with the M64 Smoke and M64 HC Smoke rounds.