They are prop guns, naturally. But even before I did some cursory internet information searches I could tell that the rifle was at least based on the M1 Carbine. I could see the bolt and slide, and the well for a magazine is only big enough for Carbine size rounds.
Rifles used in closeups were functioning, had a nice wooden, black, 'after-market' stock. They were painted black and it made them look like they were made out of metal. Rifles that weren't used or in closeups were much cruder looking, relfecting a less refined "Ape" industrial base, perhaps? But you can still see the roots of the M1 there. For such 'primitives' they sure picked a sound design.
Here is the 'functioning' rifle stock, without the Carbine parts, like the barrel and receiver, etc.
Keep yer hands offa my rifle, you damn dirty ape!
I mean, excuse me. Who am I to judge what 2 consenting hominids do in the privacy of their own Motel Room? Carry on. Love is beautiful.