View Full Version : Open Bolt?

July 30, 2008, 01:38 PM
I was reading about full auto rifles a bit and there is a lot of reference to "firing from an open bolt". Could someone explain this for me? As far as I know with my rifles the bolt must be closed to fire. What keeps the round chambered? Where the heck is the firing pin. Ive never fired a full auto so it makes no sense to me.

July 30, 2008, 02:37 PM
I'm no expert on this...but I did stay at a Holiday Inn once...

My understanding is that an open bolt refers to the firing pin being a part of the bolt itself so that when you pull the trigger the whole bolt goes forward to strike the primer of the cartridge.

July 30, 2008, 02:49 PM
Weapons that fire from an open bolt feed the rounds from the magazine as the bolt travels forward and fire upon closing the bolt, either by a fixed firing pin (slam-firing) or a delayed firing pin strike of some sort (two-piece bolt). Many submachineguns use a trigger that is just a hold-open device, when the trigger is depressed it allows the bolt to travel forward and the round fires. After the round fires, the bolt cycles back and forth and the gun keeps firing until the trigger is released. This is a cheap and very functional way to make low-pressure pistol round submachineguns, like the Sten or the M3 "grease gun".

Closed-bolt automatic firearms (like the M16 and AK47) have a mechanism to prevent them from firing before going into battery (locking) as they cycle between rounds and damaging the weapon. The closed-bolt design is typically reserved for higher-pressure rounds and/or more expensive weapons since it requires more parts to make.

David Hineline
July 30, 2008, 02:51 PM
There is this thing in machineguns called cookoff, if a round is chambered with a bolt closed just sitting there the heat from the chamber can ignite the powder the gun will cycle and load another round. If the chamber is hot enough the gun will run away with no way to stop the cycle.

An open bolt gun locks open with no round chambered. When the trigger is pulled it loads one round and fires it with fixed or hammer fired then locks open again untill the trigger is pulled again.

If the open bolt is a machinegun, well as long as the trigger is pulled the bolt cycles the gun fires, let go of the trigger and the bolt locks open allowing the chamber to cool.

July 30, 2008, 06:44 PM
+1 David, A cook off wont happen immediately as it seems, if one is "cooked off" the next round chambered will take ANOTHER 3 or 4 seconds to cook off. And if this continues, the magazine will empty. Thats all.

44 AMP
July 30, 2008, 10:22 PM
While the explantions give are correct, there is some additional information to be aware of. The manual of arms for an open bolt gun is different than for a closed bolt gun. With a closed bolt gun, when the bolt is open, the gun is safe. With an open bolt gun, when the bolt is open, the gun is ready to fire! An open bolt gun with a loaded mag in place and the bolt open is ready to go! Just the opposite of a closed bolt gun!

Open bolt guns are typically submachineguns and machineguns. Select fire rifles (assault rifles and battle rifles) typically fire from the closed bolt. There are a small munber of designs that are exceptions, but generally this is the rule. The BAR (the GI gun, not the civilian sporter rifle) fires from an open bolt, and although called an automatic rifle, it is a light machinegun. The early versions of the BAR were selective fire, but the later versions were full auto only. Another exception is the H&K MP 5, a submachinegun, but it fires from the closed bolt position.

Open bolt guns are not easy to shoot accurate single shots from. The weight of the bolt mechanism going forward to chamber and fire a round is disturbing to precise aiming, and unless mastered with practice, will tend to throw off the shot. With a full auto, this is not a trememdous problem as full autos are seldom used for firing single aimed shots. The throwing off of the aim, and the legal entanglements (open bolt guns are considered by the BATFE as too easy to convert to full auto) are the main reason the open bolt system is not used on semiauto firearms. There used to be a couple of open bolt semi auto "handguns" (the MAC/Cobray for one) but they were taken off the market and redesigned to fire from the closed bolt to get the Feds off their butts. Today they bring outrageously high prices as collector pieces, for what are, in reality, not very satisfactory guns.

The Germans came up with a novel approach with one of their guns, the Falschirmjaegergewehr (paratrooper rifle) the FG 42. A magazine fed 8mm Mauser of very unusual design, and select fire as well. Among a number of unique features is the fact that the FG 42 fired from the closed bolt in semi mode, but when switched to full auto, fires from the open bolt!

Open bolt guns are best to prevent cook off, as the airflow around and through the open chamber helps cool the gun. If you get a cookoff in a closed bolt gun, the correct thing to do is hold on to it (keeping it in a safe - downrange- direction) and drop the mag or twist and break the feed belt (or open the top cover) so the gun runs out of ammo as fast as possible.

In an open bolt gun, when you pull the bolt to the rear, it stays there, and only goes forward when you pull the trigger. I know of an accidental discharge when an undertrained guard loaded his UZI the way he was used to loading his M16! He had to pull the trigger on the UZI to do it, but he got it done!

The safety in an open bolt gun usually does not block the trigger, what it blocks is forward movement of the bolt. There are design exceptions, of course, but blocking bolt movement is the most positive method.

Hope this helps.

July 31, 2008, 04:13 AM
Thank you all for the info. As a non-auto guy that was one of the things that always perplexed me.