wouldn't know why a short rifle or shotgun wouldn't be available, shotgun is a good weapon for a new guy who keeps getting himself right in front of enemies due to not knowing the lay-out. isn't it normal to have at least one of each type available right from the start?
The only reason i said short rifles and shotguns might not be available off the bat is because there appears to only be only two of each in the game, and one of the shotguns is a heavy auto-shotty. SO it would be pretty boring if you couldnt unlock a normal shotgun, or theres only one short rifle to unlock. I suppose unlocking weapon upgrades could be just as exciting, but this is just my guess.