I did exactly that - used alginate and cast both my hands for my USCM armour display.
Since a pulse rifle is quite large and heavy I actually cast my right hand holding the grip (took 2 tries to get the index finger pointing the right direction) - then attached the whole hand/grip to the rifle. The left hand was moulded in an appropriate position for holding a motion tracker. I ended up painting both hands black to match the mannequin head.
BTW urethane resin and alginate don't work well together. Since the alginate is water based you get big blobs of water forming between the alginate and resin as the resin heats up when it cures. Plaster absorbs the water when it's setting. I used hydrocal which is stronger than plaster but if you wanted a resin cast then making a silicone mould of a plaster cast would be the way to go.
I was also thinking that casting hands in flexible urethane with armature wire down the fingers might be useful for semi poseable hands.