10 round Garand with a smaller calibre round? If only.....that actually did happen and was rejected.
StG 44 was on the way out if the war had continued replaced by the StG 45(M).
Explosive rounds was not a mass produced item.
Box magazines are not the miracle cure as an external box magazine can be trouble and the 8 round en bloc clip for the Garand was certainly fast enough and easy enough certainly over a stripper clip.
Disagree with the majority of this.
The OP's question didn't ask to factor in manufacturing cost, and that was the really big advantage the STG45 had over the 44 - also 44 certainly was NOT on the "way out", as only a handful, as in less than 100 STG45s were produced by the time Germany surrendered.
Box magazines can be trouble? Sure, any part of a rifle system "can" be trouble, however that doesn't preclude using a detachable magazine. Hence pretty much every main battle rifle in the WORLD using one right now. The primary reason I included it has already been stated by another, if you run optics, you have to offset them for a top loading en-bloc system. Optics would be the biggest single improvement to any WW2 rifle, and this is also why going to a mid range caliber in the M1 would be a huge advantage, as recoil management so far as it relates to that rifle is far better (faster) with a mid range caliber than the 30 caliber version is. IE going to a lower recoiling round would increase the performance that adding an optical advantage to the rifle would give, significantly.
I strongly disagree that the 8 rnd en bloc system was "fast enough" compared to a detach box mag would be. Try running your Garand quickly with gloves on, reloading a detach mag vs the 8 rnd clip in those conditions is laughably faster. Also, 8 rnd clips can be just as much, if not MORE trouble than a detach box mag, as it's far easier to snag them on threads/clothing/etc with the rounds being exposed the way they are, and they are far more exposed to debris getting into the clip/rounds. Again, there was a reason the M14 went to a detach box, as did every other main battle rifle from then to now. I mostly used box mag as one of my wish list items primarily to help facilitate mounting optics on every US main battle rifle so far as the OP's question went.
Explosive ammunition for rifles/ground machine guns is pretty much a needless expense and complication. AS are most things, ammunition is a compromise and HE rifle bullets require much more care in both manufacturing and handling than Ball, AP or even tracer. HE rifle bullets also really suck at penetration. Full power rifle ammo will go through 12-24 in of sand (depending on exact bullet and if sand is wet or dry), several feet of wood ( big change from pine to oak) 3-6 ins of masonry and other assorted obstacles. Little tiny bangs on the outside of such barriers can turn conclement into cover. British .303 may not have been the best at penetrating but with the nose of the bullet full of aluminium or some other filler it flopped sideways most of the time pretty quickly. A .303 bullet going sideways has pretty good stopping power
I don't disagree, but it is a "wish list" post, and considering that penetration isn't as big a concern in WW2 as today (although still a concern, granted) due to there being little to no use of body armor, I'd prefer the massive increase in lethality of any strike in the cone of vulnerability of the target over depth of penetration a ball or AP rounds gives. I agree with everything you posted regarding the increase in cost and time in manufacturing exploding rounds, but again, it's a wish list as a end user of the rifle, that's how I took the OP, if it was a based in reality list, obviously logistics is critical.
Also, IMO you can use mid caliber rounds in an MG and have it still be reasonably effective out to 3 or 400 yards. In fact the RPDs our counter assault teams used while in Iraq and Afghanistan worked at least as well or better than the SAWs we ran later on, but both were pretty effective at that range. In fact as an LMG I feel it's more optimal than a 30 caliber weapon, which typically needs as you said a decent(heavy) mount.
DARPA and other agencies are working on mid-range caliber in the 6mm-ish class right now for a replacement for both the 556 SAWs and the 762 240s, some with telescoping tech ammunition, some not, but the common thread is a mid range caliber.
Up very close, the penetration of a rifle bullet tends to be extremely poor relative to backing off 30-50 yards so the bullet has a chance to "go to sleep" before hitting the obstacle.
Could you explain this a bit more, I don't understand what you're saying exactly. The muzzle velocity of any bullet will always be it's fastest, and kinetic energy is KE = 1/2
mv2, v being velocity. Rifle rounds striking the human body at close range, the lethality often depends on angle of entry and rotation (yaw factor), but certainly it doesn't "gain" anything at 50 yards+ that it wouldn't have at 0 to 50 yards, in fact the velocity will be slightly higher at less than 50 yards, if anything.
edit -
Regarding penetration, the optimal range for something like the .30-06 is about 20-30 yards. It is enough for the bullet to stabilize or "go to sleep" but not so far as to lose a lot of velocity. I forget where I saw the results of this kind of testing but it was for US military .30-06 rounds.
Ok, thanks for the response, I understand what you were saying now. Stabilization is a pretty variable subject, as twist rate, barrel length, bullet shape, weight, composition, etc, all play a factor in that. I do agree that rifle rounds less than 50 yards can be pretty variable in the path they take in the target, but even that variability has it's own variations and set of factors.