If it converts energy into mechanical force, it is an engine. Physics 101.
Ever hear of so called "siege engines" of the Middle Ages. Maybe not suppose.
---------- Post added April 3rd, 2014 at 11:07 PM ----------
Well, maybe it was ME 101, lol. But it still stands, an engine is a "machine" which converts energy into mechanical force. In the pure engineering definition a crossbow is an engine as is a catapult, etc.
Engine - Definition and More from the Free Merriam-Webster Dictionary
Interesting that Merriams says that a synonym for motor is engine and for engine is motor which is how I'd look at it.
Merriams also says a clip is a magazine from which ammunition is fed into the chamber of a firearm. Also how I'd look at it.
Merriams may be wrong of course. As I too often am.
And yep, I know what a siege engine was/is. Reading a lot has always been one of my vices.
Once again, anybody out there that has finished an 80% AR receiver?