I assume you are talking about a firearm that fires from an open bolt.
This just refers to that the firearm using a simple direct blow back system and when cocked back the entire reciprocating system is held to the rear. When the trigger gets depressed it releases the bolt picking up a round and firing it all in one motion.
There are vary few examples that are compliant with the 1986 machine gun ban as it is a difficult system to make semi-auto. But a few famous examples of a firearm using this system is the M3 Grease gun, Stein, the Thompson, and the UZI.
The bolt is tasked with pushing the round into the chamber, holding it there while it fires, then coming back to extract the casing so the next round can be loaded
A bolt action rifle is your basic closed bolt system. If you want anything to happen when you pull the trigger then you need the bolt to be closed (forward) so the round is in the chamber and ready to go. This is basically the go to for semi-automatic designs and is considered to be more accurate because there’s less movement between trigger pull and bullet leaving
Open bolt designs latch the bolt in the back(open) while waiting for the trigger and when you pull the trigger its released and is pushed forward by springs, grabs the round, loads it, fires, and is pushed back to its locked position by gas from the fired round.
An open bolt is useful in a machine gun because it keeps the next round in a cool spot rather than sitting in the hot chamber where there’s a chance it could cook off and fire without pulling the trigger.
It means that the bolt remains open before pushing a round into the chamber and firing. The bolt remains back in the “open” position until you pull the trigger.
A closed bolt is more common, and is the opposite. The bolt pushes the round into the chamber, and *then* waits for the trigger to be pulled before striking with the firing pin.
Open bolt just mean that the bolt remain open until you fire the gun. When you fire, the bolt will go forward, push the ammunition into the chamber and lock the chamber. After the round is fired, the bolt unlock, is pushed back and will then lock open, ready to cycle again.
This is different than a closed bolt, which won’t lock back after firing. It will go forward, put a round in the chamber and then lock, ready to fire again.
The difference between the two is that open bolt will make it easier for heat to escape, making it often preferred for weapons that are designed to fire full auto continuously like most machine gun and some submachine gun (usually older design in the case of submachine gun).
The advantage of a closed bolt is that there is minimum delay between the trigger pull and the shot being fired. The bolt doesn’t need to go forward, chamber a round and lock before the shot is fire. This make the gun more accurate to shoot. In addition, since the chamber is closed and locked, there is less chance of dirt going in the mechanism. Most guns are this way.
An open-bolt system is a type of gun where the bolt is held back and it only comes forward and closes when you pull the trigger. Most guns these days are closed-bolt, where to fire the gun you (or a spring) push the bolt closed, loading a round into the chamber, and the trigger causes the firing pin to fire the round (couple different mechanisms for that, not the point). Open bolt guns are generally a lot simpler and easier to make but they’re effectively illegal in the US because the ATF says they’re too easy to convert into machine guns.
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