[ithacaartist]

Eric, I can't speak of the finer points, but generally the longer-barreled guns were issued in circumstances where a carbine would be handy. Of course, the definition of a carbine is a long gun that shoots pistol ammo. The longer models would have been more effective at longer ranges than a standard shorty, and the detachable butt-stock increased its versatility--assembled, for longer distance shots, and removed--just the pistol--for close quarters. I hope others can fill in for you how they were issued, to whom, and the reasoning behind it.