Can we mod these weapon choices?
My take, British Empire:
For the infantry rifle, start off with the .303 SMLE and a No. 5 and/or No. 6 'Jungle Carbine' in .240 Magnum Flanged in mixed service. The .240 out of that length barrel should produce ballistics similar to the 6.5mm Grendel (while the size of the case + plus the low pressure level should minimise muzzle flash, even with a deep seated bullet to match the rifle action's length. Hopefully). As the war progresses, have the larger rifle phased out in favour of the carbine.
The shortened Mark 3 Bren in the smaller calibre as section machine gun should eventually replace the larger .303 Brens as platoon/section weapons.
Yes, this means eventually no .303 at platoon level, with the exception of whatever older WWI-era rifle NCOs and officers might want to carry in that calibre. And I understand that the old deerhunting .240 is a round that's roughly the same size as .303, so its adoption doesn't mean the platoon gets to carry more ammo, as they would if they had a modern intermediate size cartridge. But you can't just go and chamber a 6.8mm SPC or a 6.5mm Grendel or a 6.25mm British experimental in a Lee action during this era, and I prefer to keep that bolt gun, in keeping with the OP 1940 baseline.
SMG: The Welgun looks like a promising weapon, and I've read it performed better than the Sten. Chambered for 9mm Parabellum as per OTL? In which case, either a longer barrel and a fixed stock are in order to make it infantry friendly; or if it's to be issued as a smallish weapon, then chambering it in the weaker 9mm Browning Long might be the trade-off necessary to prevent recoil issues. I'll handwave and say that BSA can design the Welgun at the same time OTL's Sten was thought up, right after Dunkirk (seeing as they weren't asked to in OTL, it's reasonable to assume the SOE/BSA types who designed it in 1942 could have brought the thing forward two years if need be.)
If 9x20mm is to be the standard autoloader round, then the blowback Webley and Browning 1903 designs that were available IOTL can be the service side arm.
But if 9x19 is to be the official auto round, then just chamber it in the Hi-Power for limited para and commando issue, while keeping a revolver as the main sidearm. Instead of .380 in a smaller version of the Mark VI as IOTL, I'd go with .41 Long Colt, in a reduced sized WG revolver. Colt Police .41 models can be bought from America to supplement the medium size WG, also I guess you can have 5-shot Smith and Wesson M&Ps in that calibre.
Hmmm, need a more GPMG-type weapon for company level; I think a belt-fed version of the stocked Vickers K Gun, in .303 SAA. (Considering how big the VGO receiver is, with all that room for new gas-operation stuff, then engineering it for belt feed should be a much easier proposition than with the Bren. Yes, I know about the Taden Gun, but I don't trust the ability of weapons technology to make a reliable belt-fed Bren in .303 for that era.)
Heavy Vickers .303 remain attached to battalions.
I've gone with these choices because they're not screamingly impossible for 1940-moving-forwards, but even then I think you need SMGs and small calibre rifles to have proven themselves as viable alternatives to the .303 SMLE
before the war. Perhaps during the Imperial brushfire skirmishes of the 20s and 30s.
It would be cool to say 'select fire 7mm bullpups for everyone!!11!!!', but that's not happening.
