The definition that I seem to find mostly consistant (if a bit vauge) is that it's in-between a pistol round and a full-sized rifle round.
The logic behind this was that despite fielding rifles firing full-sized cartridges out to distancs otu to a thousand meters or so, most battlefield engagements were taking place at ranges far smaller than that, 50-300 meters. More urban combat, rather than across the no-mans-land in WWI. You can blame myopic military minds for this; why get rid of al those sturdy rifles and mountains of stockpiled ammunition left over from yesteryear, if it all still works?
(The answer is that the battlefield conditions and tactics made them obsolete, no matter how handsome and sturdy they were.)
Cue lots of submachineguns being manufactured and fielded. However, an SMG is typically worthless beyond 200 meters (same with the M1 carbine, which is firing a long pistol round). Amazing for close-ranged affairs, not so much for open field condition, which are still something to worry about. Plus, lack of kinetic energy, as well. Pistol cartridges just don't pack the oomph that a rifle cartridge will, at long ranges, even if you do manage to hit them with your pistol cartridge.
So, the solution was to give the soldier something that could reach out and touch someone at rifle-like engagement range, while still being capable of controllable high volumes of fire like an SMG - full-sized rifle cartridges (like a 30-06) out of a small firearm is very hard on the parts and difficult to control in full automatic mode. The German-designed Sturmgewehrs (assault rifles) did this job well, but were introduced in insufficient numbers and too late in the war to make much difference in the final outcome of the war - they were still spending resources making 98K's, a rifle that had essentially served the Gernam Army for half a decade at the time, in one form or another.
Even as early as WWI was some work done in this area - change the bolt out on your bolt action, and insert a simple SMG action into its place, and you've converted your .30-caliber bolt action rifle into a .30-caliber long-barreled blowback-operated submachinegun. Pederson Device, I think it's called. Amazing concept, didn't work too well in the field - it approached the problem differently than designing an intermediate rifle and cartridge.
edited 27th Sep '10 12:35:32 PM by pvtnum11
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