We know that now, because of the bomb. Had we been forced to invade Japan, those would have made a huge difference and not distributing them was a failure on the part of the Axis leadership (one of many).
By 1944, Germany and Japan were pretty much finished. Up against the almost limitless manpower of the Soviet Union and the production capacity of the United States, sooner or later defeat was, essentially, inevitable. More importantly, the German submarine campaign had pretty much failed by that point, while the US was poised to finally bore in on their own submarine interdiction efforts in the Pacific.
We didn't have to drop the bomb or invade Japan. Starving them out through a total submarine blockade, which by 1945 we'd accomplished, would've eventually defeated them.
Of course, this is all hindsight.
And from the German perspective, look at the kill ratio of Germans to Russians in 1941 & 1942, when the German army was basically armed with bolt-action Mausers, imagine the damage they would have caused armed with even a decent semi-automatic rifle.
Most battlefield casualties in WWII were, if I recall correctly, caused by artillery, not small arms fire. Firepower per infantry squad is certainly an advantage tactically, but I think you're overstating things a little, especially against the Russians.