I just can’t see the cash strapped 1930’s Army adopting a new rifle cartridge that requires also refitting all of their automatic rifles.
They were planning on it with the .276 Pedersen until they realized they could still use the .30-06 in a semi-automatic rifle. That meant they were prepared to adopt a new auto-rifle chambering as well.
The .25-3000 round, designed in 1915, was produced by 'Savage' company.
Here we have the designer and manufacturer have no problems with US Army using it, as long as they have a sizable contract for supply. I'm not suggesting this cartridge as be-all end-all, just picking up something available in the period the self-loading rifle was being designed for the Army. Nor I'm suggesting that ww2 will end in Allied victory by 1943.
After the lenghty intro:
- what weight and length we can expect for the ALT M1?
- scale of issue?
- possible spin-offs before 1945?
- earliest automatic weapon with this round?
- M1 Carbine is still developed?
- what might be reasonable/feasible answers by Germany, Japan, Italy, UK, USSR once they get good data on the rifle, and/or acquire the examples of it?
- post war development & spin-offs of both infantry firearms and cartridges?
The .25-3000 does not to have fire at actual 3000 fps, I'm okay with a bit lower MV when heavier bullet is used.
Edit: sorry, though this was 6mm, not 6.5mm.
The .25-3000 nearly matches the requirements for a 6mm military cartridge set out in the 1990s by Stanley Crist in the Infantry Journal:
http://www.g2mil.com/6mm_optimum_cartridge.htm
One caliber can do it all. By sending a 100-grain, very low drag bullet downrange at over 2900 feet per second, the 6mm Optimum would provide snipers with the flat trajectory of the .300 Winchester Magnum. It would give machine gunners the penetration potential and tracer capability of 7.62 NATO, thereby permitting the development of an infantry machine gun that is light enough to replace both the 7.62mm medium machine gun and the 5.56mm squad automatic weapon. And, by virtue of being a compact, lightweight cartridge, with low recoil impulse, it should also allow the creation of a combat rifle that is little or no heavier than the M16A2.
https://en.wikipedia.org/wiki/6mm_Optimum
Crist's specifications were that "6mm Optimum" ammunition achieve, with a 100 grain bullet:
- velocity: 2,900fps (muzzle), 1149fps (1,200m)
- energy: 1,867ft-lbs (muzzle), 293ft-lbs (1,200m)
- flight-time to 1,200m: 2.21 seconds
- deflection @ 1,200m in 10mph crosswind: 151 inches
- maximum trajectory: 244 inches
...of which he noted,
"...even with a conservative estimate for the muzzle velocity of the 6mm Optimum cartridge, computed data for 1200-meter velocity, flight-time, wind-deflection, and trajectory height are all greatly superior to both 5.56 and 7.62 NATO rounds."[1]
https://en.wikipedia.org/wiki/.250-3000_Savage
Newton recommended loading 100 grain (6.5 g) bullets at 2800 ft/s (850 m/s)
With a more modern powder achieving 2900 fps muzzle velocity is really no trouble.
Even with powders of the period with a 24 inch barrel they could get up to very near 2900fps:
100 gr (6 g) SP 2,864 ft/s (873 m/s) 1,822 ft⋅lbf (2,470 J)
Test barrel length: 24"
Source(s): Accurate Powder [1]
The Garand had a 24 inch barrel IOTL:
https://en.wikipedia.org/wiki/M1_Garand
Barrel length 24 in (609.6 mm)
So effectively the .25-3000 is exactly a '6mm Optimum' cartridge in the M1 Garand. You could certainly have a lighter rifle, as the M1985 Lee Navy, which fired a relatively similar cartridge was about 3.77kg:
https://en.wikipedia.org/wiki/M1895_Lee_Navy
The difference though in the long stroke gas piston system would certainly up that, but with a 4 inch shorter barrel the Garand would save a bit of weight there.
So a 4kg M1 Garand should be easily do-able, which means a rifleman could carry at least double the ammo load compared to OTL.
Not sure the Johnson LMG would be the right way to go with the side mounted magazine, but something like it would be better than trying to make due with the BAR. Not only would it weigh a lot less, but would be cheaper to make.
Of course that's assuming the Garand isn't made select fire and magazine capable, which might well make an automatic rifle/magazine fed LMG redundant to the American rifle squad. With a 4kg rifle the recoil of the .25-3000 cartridge should be very mild, especially by the standards of the day, which would make automatic fire very controllable. The only issue would potentially be overheating.
However to have something so actually ideal would require a lot of hindsight, as the US army had already killed the 6mm Lee Navy based on faulty assumptions about future needs. But handwaving that, the M1 Carbine might well never be developed and instead a carbine version of the M1 Garand would be viable:
https://www.luckygunner.com/lounge/m1-tanker-garand/
I doubt the Axis powers could do much with matching it considering that they were already in a shooting war and caliber switching was impractical. I guess maybe the Germans could try something in 6.5mm given how many nations they occupied that used rifles in that caliber. The existing 7.92 Mauser already had a development that mounted a 6.5mm bullet on the necked down cartridge, so you could rebarrel existing equipment to work with it. With a 100 grain bullet though it would be pretty damned hot, as the 6.5mm Swedish cartridge shows (the ballistics matches that of the 6.5mm Mauser) as it had a 970m/s muzzle velocity with that weight of bullet. They'd probably have to shoot for a 120 grain bullet to get it down to around 850m/s and have a better sectional density. But I doubt they would bother with all that...still, thinking about the FG42 in 6.5mm Mauser is pretty interesting.
Post-war they'd probably want a belt fed in this cartridge and would probably upgrade the bullet design to improve aerodynamics. Not sure if there would need to be a spin off other than an upgraded rifle with inline stock design and pistol grip, plus select fire and magazine feed if it didn't already have it. No real need for an automatic rifle version, while the belt fed could be as light or lighter than OTL SAW.
