Quoted:
No, the AR15 as we know it today couldn't have been mass produced back then.  Could it have been designed back then?  Of course.  Browning is generally credited with using gas from a gunshot (via a hole in the barrel) being used to cycle a firearm circa 1885.  The concept of an intermediate sized cartridge that filled the gap between pistol bullets and rifle bullets goes back to the end of the first world war.  It was dismissed.  The M1 Carbine round was kinda, sorta, but not really in this class of thinking.  David Williams was still thinking in terms of caliber over velocity as were firearms designers of that era.

The AR15 started life as an AR10 firing a 308 Winchester round (some Vietnam vets called this the 30-06 Short).  It ripped off the Browning design of 1885, ripped off Melvin Johnson's bolt head design and added a gas pipe (rather than a forward piston or operating rod system) to actuate the weapon upon firing.  Additionally the AR15 ripped off the STG 44 dust cover and reversed it to come down instead of up.  Direct impingement was already being used in 1940, but not exactly as Stoner/Sullivan used it.  The AR10/15 basically moved the piston system all the way back to the bolt (thus introducing filthy gasses DIRECTLY into the major moving component of the weapon system).

The same reasons that caused the rejection of the AR10 when it was introduced would have caused the rejection of the AR15 in 1942.  It would have had to be made from steel, because aluminum would not have truly been considered.  In those days it was steel and wood, not aluminum.  It would have had to fire the 30-06 round.  Ten pound full auto 30-06 would have suffered from the same disease that full auto 308 AR10's did when tested in the 50s.  The M14 full auto was a failure as a true combat weapons system.  The intermediate cartridge just hadn't come of age yet in 1942.  Even Hitler refused to accept it and his designers were the first to think it up.

So it could have been designed, but its highly unlikely it would have been accepted.  Additionally getting a 22 caliber weapon accepted as a combat rifle would have been laughed out of the building at that time.  It was laughed out of the building when it was introduced later on.  There is still plenty of debate today as to the effectiveness of the 5.56 vs the 7.62mm.  Small arm military combat calibers have shrunk considerably since the late 1870's.  In the American Civil War we generally used 58 caliber Minie balls and now we are shooting 22 caliber high velocity rounds.  People figured out that penetrating a vital organ is what it take to kill or incapacitate an enemy combatant.  War costs money and its far more effective to give a solder 240 rounds of killing power than it is to give a soldier half that capacity of a larger caliber to do the same job.  Firepower counts in a firefight.

Quoted:
The AR-15 is an absurdly simple firearm compared to an M1 Garand or an FG42.  It would have been quite easy to produce.

Quoted:
So little actual knowledge being used to make erroneous statements.

Something to think about that contributes to the technology of the M16.
https://www.youtube.com/watch?v=hpgK51w6uhk

Quoted:

Very cool video and well worth the watch.

Thanks!

Quoted:
Johnson Automatic Rifle bolt:

http://www.practicallyshooting.com/wppshp/wp-content/uploads/2011/11/IMG_1467.jpg

https://www.picclickimg.com/d/l400/pict/273471097828_/Model-1941-JOHNSON-SEMI-AUTOMATIC-RIFLE-BOLT-%C2%A0.jpg

Quoted:

Oh hell no! We didn't have the advanced technology needed back then for such complicated weapons systems. OP, you're being foolish.

On a completely different subject, I'm a big fan of some of rudimentary and archaic 3,500 horsepower supercharged engines they used during the war. Amazing that any of those planes even got off the ground if you think about it.

https://i.pinimg.com/originals/90/3c/1b/903c1b12f47d81e6afbf745faf9fd6d0.jpg

Quoted:

Oh hell no! We didn't have the advanced technology needed back then for such complicated weapons systems. OP, you're being foolish.

On a completely different subject, I'm a big fan of some of rudimentary and archaic 3,500 horsepower supercharged engines they used during the war. Amazing that any of those planes even got off the ground if you think about it.

https://i.pinimg.com/originals/90/3c/1b/903c1b12f47d81e6afbf745faf9fd6d0.jpg