Upper Bolt Venturi Mod
The upper bolt of a stock VM-68 is a good piece. It utilizes 4 o-rings which prevent gasses from escaping up into the feed elbow and preventing ball breakage. This also helps in overall velocity.
The only drawback to the stock bolt is the flat ball surface. When firing, the paintball is pushed into the barrel under much pressure over only a small surface area of the ball. This tends to break paint.
The two improvements that can be made include venturi porting and weight reduction. Machine work can be done on your stock VM bolt that will cradle the paintball in a uniform fashion while firing, thus reducing ball breakage and increasing accuracy. The bolt work will also reduce the weight to improve cycling.
Aftermarket venturi bolts run over $30, and many do not utilize o-rings, or have inner tapers that hamper ball feeding and have excess gas blowby reducing velocity. Stick with a modified stock bolt for best performance!