A worn sear or rear bolt will make the gun 'rapid fire' like a machine gun, but too fast to work properly. The gun may not stay cocked either. These are both caused by the rear bolt slipping off of the sear that is supposed to catch it and keep it back in the cocked position. If either your sear or rear bolt is worn, Tippmann suggests replacing both at the same time. A worn sear will wear down a rear bolt faster than normal and vice versa. Replacing only 1 of them may result in having to replace the same part again a lot sooner.