Cupped Face Bolts
Many people believe that a cupped face bolt is good because it "cradles" the ball. What this does, however, is to allow the ball to roll back in the breach slightly which allows the ball stack to settle lower. The result is that when you fire, the ball in front of the bolt moves forward and bounces the next ball in line out of the way causing the whole stack to jump. This can cause broken paint and misfeeds. This isn't as much of a problem with closed bolt guns (Cockers, Blazers, etc.) as it is with open bolt guns (VMs, Spyders, Tippmanns, etc.).