A semi or fully-automatic firearm which is said to fire from a closed bolt is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward. When the trigger is pulled the firing-pin or striker fires the round, the action is cycled by the energy of the shot sending the bolt to the rear which ejects the empty cartridge case, the bolt then goes forward feeding a fresh round from the magazine into the chamber, ready for the next shot.

Comparison with open bolt design

Pros

• More accurate for the first round:
  • No movement of working parts to inhibit accuracy.
  • Round seats consistently in the chamber.
• Action can be locked forward to further reduce noise in a suppressed weapon.
• Can carry an additional round in the chamber, increasing ammunition capacity beyond the magazine's limit

Cons

• More complicated and expensive to manufacture.
• Less heat dissipation from closed chamber.

Closed bolt weapons

• HK MP5
• FNAB-43
• Mini-Uzi

Mixed mode weapons

• FG42

See also

• Open bolt