I've had some of these in my hands, also, and my experience leans in the same direction than archer's. The ones I've seen have a mechanism to "cock" the firing pin and some kind of "trigger" to release it. They are not always obvious, but can be found with relative ease by fumbling around with any seemingly loose bit. Keep in mind that they CAN be deteriorated and not able to operate enymore.
By the way, and just as a general advice... Do NOT try to fire them. The conversion from weapon to conversation piece is frequently done poorly and with too many concessions to aesthetics and concealability. They can be as dangerous for the operator as for the target, if not more.
They're pretty cool, though...
