Closed bolt: When you pull trigger, the hammer hits the firing pin, causing the primer to ignite (ar-15, etc) Open bolt: Pulling trigger results in the bolt going foward. The firing pin goes foward automatically when the bolt is all the way foward. (m60, etc)
[Last Edit: 4/25/2006 5:13:36 PM EST by olds442tyguy]
Open bolt has a fixed firing pin, where as closed bolt is hammer struck.
Say you had an open bolt handgun. When you fired a round off, the slide would lock back as if you had just shot the last round. When you pulled the trigger, the slide would release and the fixed firing pin would strike the primer as the slide came forward.
As I recall, the primary reason for an open-bolt action on sub- and machine guns is so that when you let go of the trigger, the chamber is empty & the bolt back; that way, after firing for extended periods, the risk of ammo "cook-off" is greatly reduced.
Open bolts designs are most common with Sub machine guns and Belt feed weapons due to their Cyclic rate and the heat generated by the rate of fire. Open bolt designs keep a round out of the chamber until the firing sequence is intitated thus reducing the chance of a chambered round from cooking off in a hot barrel/Chamber
The Only Large Caliber MG that fires from a closed bolt is the M2 BMG.