Because several Armory weapons aren't bound to a weapon slot, many users have had a problem using them in-game. I'm posting these instructions to nip this problem in the bud.
To select the weapon, drop to the console, and type "use SummonName", where SummonName is the actor name for the weapon (what you would use for the summon cheat). If I wanted to select the Plasma Rifle through the console, I would type "use plasmarifle", and if I wanted to use Heretic's Dragon Claw, I would type "use blaster".
I hope this helps (and more importantly, cuts down on people asking about it.