..50 Browning HMG is too damn big for Humans ot ever use except on a tripod or the like, so no use for Fallout unless they put them in non-movable mounts...or for super mutants

You wanna lug an 80 POUND weight gun with recoil that will knock you flying or bust your bones?!
Bren Gun and Sterling would rock! (though the Sterling as we know it wasn't used in WW2...though I think some early models were used at Anheim?)
Bren = light machine gun with almost rifle accuracy and fast magazine change, reliable as heck. Should have accuracy (spread) problems though when not crouched as it wasn't meant to be used that way (should only be fired lying down using bipod) though it was often shown in dramatic poses being fired from the hip...which ruined the point of it's great accuracy.
Was used until quite recently by British Army (and many did not like it's crappy replacement, blech), and the Bren is still used here and there across the world.
Sterling = one of the few guns with a large magazine you can easily fire lying down and well, it's cool

Tell you one I'd love to see that few folk have heard of:
The DeLisle Carbine .British secret weapon developed for the SAS, SOE and similar units to snipe German officers/sentries silently.
Modified Lee-Enfield bolt action rifle, changed barrel etc, for a much shorter, very thick silencer/barrel, and fires .45 Colt Auto ammo, and even used the Colt .45's magazine, so it would use current Fallout ammo.
Point was, the.45 is a sub-sonic round with hell of a "knock down" effect (about a 90% chance of one hit from a .45 killing you or rendering you
hors de combat which is heck of a lot better than a 9mm etc),
so it was perfect round for a silenced weapon, but pistols are very innaccurate (forget the films, most folk can't hit jack squat after 15 or 20 yards with a pistol..and frequently miss even point blank)
So, the British made a carbine, shorter than a rifle, easier to carry and conceal, but much more accurate than a pistol.
IIRC modern built DeLisle clones have about 2" inch spread at 200 yards, perfect for pot-shotting sentries or officers from a hedgerow or roof

http://en.wikipedia.org/wiki/De_Lisle_carbine