FWIW, I found it very difficult to find suitable projectiles for paint ball guns. Mentos used to fit perfectly, I'd leave them in the wrapping, and cut 3 off, and manually loaded, they worked well. Then, Mentos changed size.

I've been to every craft store looking for suitably sized nylon balls, no luck.

I replaced the hammer spring with a beefier one, took a little off the valve return spring, and from the CO2 connector, to the bolt, opened up the ports a little.

The problem, is that past a certain velocity, the paint balls vaporize in the barrel. I tried freezing them, but they never fly straight, as the paint freezes in the bottom of the paintball, causing it to be imbalanced.

My solution was to fabricate a smaller bore barrel insert, and use marbles. I had to make reducers for the hoppers, and make a new ball stopper where they chamber, (otherwise if you lowered the muzzle, being open bolt, they would all roll through the barrel due to gravity).

It worked well. Electronic selective trigger group, (S 1 3 A) and finished. Puts marbles through both sides of a Gyprock wall now. It wont penetrate a car door, but leaves significant dents, and devastates auto glass.

Still smoothbore, so forget about long range accuracy.