If you want it to last, yes. I don't think the motor is at risk, though it will deteriorate faster than in other applications, the risk will be in the gearbox. Slamming a weight back and forth, plus having an olvervolted motor, I can't see it lasting long.

If it's a 36:1 gearbox you could make it 18v and change to 24:1. You should still have loads of torque to swing it round and you'd have a more reliable setup, whilst making it faster than it is at 12v in those tests.

edit: you may find it lasts longer if take some weight out of the weapon head, too. If it's a kg or two then the momentum behind it may just destroy the gearbox when you use it in anger.

Perhaps before you do anything you just build it as planned. If it breaks, you have options, if it doesn't, you have a powerful weapon.