[Travis McGee]

Depends on how you define successful, I suppose. First invention beyond a working prototype? Of first wide introduction to a military? Probably the Thompson SMG (not a rifle, but a long arm none the less), or the Garand and or the M1 carbine. Archy will know!

[sport]

I think that I read[or heard] that both the BAR and Thompson were developed toward the end of WWI. The BAR to augment the firepower of the rifle squads and the Thompson for trench fighting.

[MileHi]

I would say successful means issued in some numbers as a standard troop weapon. I would have guessed the M1 rifle, but that seemed too easy. Thought there may have been something before that, even if it had a limited service life. I could not think of anything from the russians, germans, brits or french. So maybe the first was indeed the fabled Garrand.

The Thompson might be correct though I was thinking more of the standard troop rifle.

Of course, archy would know the nuts and bolts.