If your paintball marker chops your paintballs, stop what you are doing right away and clean it out.  Clean your gun thoroughly where the ball broke.  Clean the bolt, bolt face and the marker barrel.  Make sure to clean out all of the paint as well as the film, shell bits, and any other debris that does not belong there.  If you continue to chop paintballs, take the gun apart and check for damage.  Completely disassemble the marker, and check out the bolt face for burs, nicks, chips, and other damage.  If you have found any damage to the bolt face or o-rings, then you should replace the damaged components.  If you weren't able to fix the problem while at the field, when you get home take your paintball gun apart and inspect all of its components for scratches, fraying, or other damage.  Check out each individual part.  As soon as the paintball gun starts to fail or act funny, try checking the sear and hammer first.  For your marker to operate flawlessly, the hammer and sear must be sharp, with cleaning edges that are not worn down or worn out.  If your bolt, sear, and hammer do not show any obvious signs of wear and tear, have an airsmith inspect the entire machine. An airsmith is to a paintball gun what a blacksmith is to machine tools.