What is open bolt firing?

Explanation at bottom [url]http://www.ar15.com/forums/topic.html?b=3&f=118&t=161969[/url]

BTW Its not polite to post a topic with the EXACT same name of one that is still active.

It is a mechanical system used mostly in cheaper sub-machine guns.  When you pull the bolt back and cock the gun the bolt stays back.  When you pull the trigger the bolt is released, it fly's forward and strips a round from the mag chambers it and discharges it immediately.  In full- auto mode the gun does this until it the trigger is released at which point the bolt locks back again (in the open position), or the mag is emptied.  In this system the firing pin could be (I guess) fixed in the forward or protruding position.  A hammer may or may not be necessary as the bolt also acts as the force to detonate the bullet.  The open bolt system is  much simpler and has less parts, but when the gun is cocked you have a...well open bolt so dirt and junk can just fall inside the gun.  I hope that was not to confusing.          

While the mechanical explanation is quite correct, the statement that it's a system used in "cheaper submachine guns" isn't quite accurate.  In fact, it's used in most all submachine guns, with just a couple of exceptions (H&K being one), and in ALL full sized machine guns. If the bolt stayed closed between trigger pulls in an M60 or a Ma Deuce in between trigger pulls you would almost certainly get "cook off" rounds being set off by the extreme heat in a barrel that just had 50 or 60 rounds thru it in 5 or 6 seconds.

Think of it this way.  Open bolt for full auto guns; closed bolt for aimed fire from semi autos.  There was one WW2 piece, the Johnson Light Machine Gun, M1941 and M1944, used by the Marine Raiders, that in fact would fire closed bolt from semi auto, and open bolt from full auto.  The Marines found it a mixed blessing[:D]

You guys forget the best one of all, Cal 30 Browning Automatic Rifle, commonly refered to as the BAR, and that heavy Mother..... by the guy who had to carry it.

The Brownings are all closed bolt designs, including the big Ma Deuce.

The cook-off problem can still be a problem with an open bolt design.  If you end up with a live round in the chamber, with the bolt locked back, it will still cook off.....and the breech will be unlocked.

Um. The gun doesnt chamber a round [b]until[/b] you pull the trigger.

Well, that is the way it is suppose to work.  

But if you have a malfunction, whereas the round does not fire when the bolt closes (bad primer perhaps) and then the round fails to extract when you cylce the bolt back, you my friend, are in deep shit when your open bolt gun cooks off with the bolt back.

Yup. One thing my little bro pointed out since I posted that was that sometimes the receiver heats up so much that the round might cook off while its chambering. Extremely rare but not impossible.

Forget HELL! I had to carry one[:D] (Yes,I'm that old)  In the brown shoe army, if you were 6' or more in height, you were a prime candidate to lug that M1918A2.  Hated to tote it; loved to shoot it.

My 1919a4 in .30 fires from a closed bolt.
My M9/11 fires from an open bolt.
I handled an M60 once and it fired from an open bolt.
I've seen a lot of photographs of Thompson .45"s in combat and they appear to fire from an open bolt.

Again. According to my little bro (hes the m gun guru in the house) Thompsons do fire from an open bolt.

The German FG42 rifle was like this also.  Closed on semi, open for full.  It's alos one of the parents of the M60 (FG42 gas system, MG42 feeding mechanism)