An open bolt gun is one in which the bolt locks open, as the name suggests, until you are ready to fire.
So, in the firing condition the gun is like this, with the bolt back.
When you pull the trigger, the bolt goes forward, picks up a cartridge,
and fires it as soon as it is fully closed.
The alternative to this is a closed bolt firearm, in which the bolt is already closed
when you pull the trigger.
There it's ready to fire, and then it does fire.
The question of open bolt versus closed bolt has no necessary relationship to whether
a gun is fully-automatic or semi- automatic. As well as no relationship
to whether it has a locked breech or is a simple blow-back action.
We typically see open bolt guns on full-auto firearms, machine guns, but not always.
Here is an example of a fully-automatic, open bolt gun that does have a locking breech.
You'll notice the rotating bolt there. When the bolt closes it rotates to lock, and then it fires.
As opposed to the submachine gun you saw earlier, which is a simple blowback system.
We'll touch on the difference between locked breech and blowback in a future video.
Both open bolt and closed bolt systems remain very common today, with open
bolt guns being typically fully-automatic machine guns in a variety of calibres.
Anything from submachine guns up to heavy machine guns.
The advantage that an open bolt system brings to a machine gun is that it does not
leave a cartridge in the chamber. So, when one is not actively firing, the next cartridge
to be fired is sitting in the magazine, and not in the actual chamber.
In a fully-automatic firearm the barrel, and thus the chamber, can get very hot,
potentially to the point of being hot enough that if you leave a cartridge sitting in there
it will heat to the point that the powder, or the primer, spontaneously combust
and fire without the trigger being pulled. That is called a cookoff, and it's
a substantial potential problem for a fully- automatic firearm, particularly one that is belt fed.
In addition, having the bolt remain open like this does leave both ends of the barrel
open and allows a little bit better airflow to help cool the barrel, although that's a secondary consideration.
You might wonder why all firearms aren't open bolt designs if they have this advantage.
The reason is that it is more difficult to accurately shoot an open bolt gun,
because from the moment that you pull the trigger on an open bolt gun you have
this entire mass of the bolt moving forward, locking (if it's a locked breech gun)
and then firing. This gives a lot of time for your aim to adjust, to slip,
and it also means that you have this impact when the bolt actually fully chambers
the cartridge. That's going to jolt the gun, and again cause potential problems with your aim.
On a closed bolt firearm like this, the bolt is already fully forward with a cartridge
chambered when you are ready to fire, and so pulling the trigger doesn't cause any
heavy parts of the gun to move, just the hammer or the striker.
This makes it more accurate as a practical matter.
In addition to the practical concerns about shootability, there is also a factor
of open bolt firearms are typically simpler, easier and cheaper to manufacture.
And so there are also a class of semi-automatic, typically, open bolt sporting forearms
produced simply because they were less expensive, like this .22 calibre rifle.
In the early 1980s the ATF in the United States came to the determination that semi-automatic,
open bolt firearms were easily convertible into machine guns, and thus
issued a ruling that semi-automatic, open bolt guns would no longer be legal to manufacture.
Guns that existed at the time of that decision, like this one, were grandfathered in
and may be owned and sold and bought and traded in the United States,
but new production of semi-automatic, open bolt firearms are prohibited.
This is not necessarily the case in the rest of the world, as there are many countries
in which semi-automatic, open bolt firearms are treated no differently than
semi-automatic, closed bolt firearms under the law.
Hopefully that has answered some of your questions about the difference
between open and closed bolt firearms operation.
I'm Ian McCollum with Forgotten Weapons. Thanks for watching.