3.5lb carbine requirement instead of 5lb?

[yulzari1]

If you reverse the OP concept then you are looking to save a total of 1,5lb. Nice but is it worth the effort? 50 rounds of 9x19mm will bring the target down to a reduction of 1,35lb compared to .30 carbine. If you are lightening an M1 then one could go to 9x19mm and shave mass off the existing design and, with the excellent sights, cope fine out to 200 yards. Shorter barrel and throw away the bayonet, shorter receiver and job done plus one less ammunition type for the allied logistics.

IOTL one has to question why the USA had both the M1 Carbine and the M3 SMG. plus loads of M1911 pistols. They were for the same users. Ditch the M3 and swap to a 9mm pistol and .45acp can be abandoned completely. If the argument is the M1911, Thompson, M3 and M1 were needed to have enough PDWs then the answer is to make Sten guns and abandon pistols.

 

[marathag]

swap to a 9mm pistol and .45acp can be abandoned completely. If the argument is the M1911, Thompson, M3 and M1 were needed to have enough PDWs then the answer is to make Sten guns and abandon pistols.

.45 ACP might as well been been Holy Writ in the US Army at this point.
No way to get 9mm(or 38 Super or related cartridges) past Army Ordnance in Pistol or it's outgrowth, SMG
the M1 Carbine started life as a 'Light Rifle' so not bound by the above Commandment

 

[marathag]

Hmm.
M1 Carbines were converted to 45 Win Mag in the 1980s, never was able to find one for my collection.
Used same magazine, but acted as a single stack, not double like the .30 Carbine

Green line is 45 ACP with higher velocity for Magnum load. 1072J Energy

So M1 Carbine in 45 Win Mag has similar energy, but worse ballistics than .30 Carbine, as expected
But none could dare criticize the stopping ability of the blessed .451 dia bullets

 

[phx1138]

So I'd imagine something like this. Standard .45 Auto, .38 Super or if you need a bit more punch, .45 Remington-Thompson (.45ACP with the case extended by about 1/8"), which is close to the mostly-forgotten .45 Winchester Mag.

With something as potent as the .45 RT or Win Mag, you are going to have issues of frame battering and potential cracking. That requires either stronger frames or lower-pressure ammo. I'd suggest something more like a .38 Super +P (not the current +P, but something with actually greater pressure and velocity) that pushes the edges of the .38 Super round without unduly stressing the frame.

I'm not sure a .30 wouldn't be a better idea, for the possible flatter trajectory (something like a .30/.38 Super or .32/.38), but I fear the chamber pressure in such a round would be prohibitive.

I also suspect the 150 yard spec is going to be considered too short... If that's all you want, why not just adopt the Thompson in .45 RT instead of .45 ACP?

like it was the .38 Super, let alone the .375 Magnum

The .38 Super's actual ballistics exceeded those of the .357's... The notional advantage of the .357 was in an unvented test barrel, not in the field.

That's not counting the issues of chamber pressures requiring a stronger weapon, and feeding of a rimmed round in a semiauto (let alone full auto).