Agreed with Punisher068, the Splatmatic Flying Colors paintballs are better for the 3357, it is a bit hard on paint, though my rebuilt/restored ones I sell work better on brittler paint.
I've tried all kinds of different .50 caliber paint in 3357s, (Formula 13, GI Milsim, Kingman Slate, etc), and they all seem to break easily in the 3357. The trade off with the harder Flying Colors, which may not break in the gun, is that it is harder to get it to break on target. You might have to hit an actual hard target, such as mask lense, pack, hopper, gun, belt buckle, etc).
I always include 100 rounds of Splatmatic Flying Color paintballs with the 3357s I rebuild and sell, in fact I have two for sale right now:
And it is pretty awesome when I get an elimination with the my two 3357s (after the game, the eliminated player often has to ask me just what the heck I shot them with):