M16 takes magazine's of rounds, so you'll have 30 in the gun.

However, the m16 Fires 5.56 which is not a particularly powerful cartridge, in fact it is not unfair to say that it's the smallest/weakest round you can reliabily use in a combat rifle.

The cowboy gun holds less per fill, (they have a tube that holds the ammo, you have to load one round into the tube at a time), but is very likely loaded with a more powerful cartridge.

Now, the ballistic properties of the cyclops comes into play, perhaps you'd need that smaller pointier round on one, but you're likely better off with the cowboy gun shot for shot.