View Full Version : AR-15 firing from an Open Bolt

11-11-2007, 5:14 PM
I remember seeing this somewhere, but I can't exactly remember what kind of AR it was. I believe they were firing in full auto if I recall correctly. What would be the advantage of such a design feature?

4 Brigada
11-11-2007, 5:18 PM
What would be the advantage of such a design feature?

That I can think of none, are you sure about this being done on purpose? not a malfunction like a slamfire?

edited later

It would take a lot of redesign work, probably someone who would know would be the BATF

M. Sage
11-11-2007, 5:39 PM
For full-auto, to prevent cook-offs.

Whether or not it's FA, an open-bolt firearm is considered an MG.

11-11-2007, 5:41 PM
Its called the M231 Port Firing Weapon.
It is a full auto machine gun that mounts inside the Bradley fighting vehicle. Its purpose is to mow down any enemy dismounts that get to close the track.

11-11-2007, 5:43 PM
The LWRC Gas Piston IAR is the only one i can think of. can see it here:


Its able to switch between open, and closed bolt when used on auto, or semi, respectively. Correct me if i'm wrong, Open bolt operation is useful on MGs because it helps keep the barrel cooler for longer duration and speeds up changing barrels when they get hot in the middle of a fight. Not quite sure how this helps an AR however, unless they have a quick on/off barrel feature. LWRC is good stuff.

11-11-2007, 6:00 PM
The colt light machinegun uppers that were produced with the heavy barrel and those square funky looking handguards was sold as an open bolt and came with the open bolt fire control group and boltassembly. Pretty much all the kits sold had the auto componets removed from the boxes.
I have seen the FCG and bolt assembly pop up fro sale on a few sites and they sell for colse to $2000.

Like it has been stated it is to allow the weapon to cool off a little between shots and so there is no cartridge sitting in the chamber that can cook off if the weapon is hot.
Sometimes it is done for cost reason and for ease of production like most of the ealry submachine guns.

11-11-2007, 8:15 PM
FA weapons also put a heavy beating on their parts, especially the FCG and bolt parts. Using an open bolt allows for a fixed firing pin which reduces the ammount of moving parts. It also does not need a hammer as the inertia of the bolt from spring tension sets off the round, which leaves less parts to malfunction or get damaged. Also, the first shot off takes relatively the same amount of time as the hammer falling to the bolt, and the subsequent firing pin to cartridge contact is roughly the same time frame as the bolt flying home, maybe a few thousandths of a second faster but not much; however the open bolt design fires as soon as it hits home where a closed bolt design hits home and must wait for the hammer to be actuated, fall, and repeat the process. This is why open bolt SMGs run about 300-400 rpm faster than closed bolt variants. This also increases muzzle climb drastically so it's a trade off.

For non-NFA items there is no real need for it.

11-11-2007, 8:44 PM
For full-auto, to prevent cook-offs.

Whether or not it's FA, an open-bolt firearm is considered an MG.

This is not true, not all open bolts are MG's. There are a lot of semi-auto open bolt rifles that were produced before the ban in 1986. They are still legal to transfer they just cant be produced anymore. A lot of ingram and mac pistols used to be made as open bolts. There is also a few open bolt 22's like the marlin model 50 (thanks ajax) and there is also the gevarm that is an open bolt.

And that is one cool rifle. The way it can switch between an open bolt and a closed bolt, it would be cool to be about to see how they lock the firing pin forward, or if they just have it as an open bolt then it slams forward and there is still a hammer to hit the firing pin to make it hit the primer. Way cool gun.

11-11-2007, 8:46 PM
LWRC...that's the video I saw! Thanks guys! :)

ETA: And thanks for the info as well.

11-11-2007, 8:51 PM
its funny, they are using a german song in that vid.

11-11-2007, 9:53 PM
.50BMG fires from the closed when the bolt release is latched down.

Cav, did you ever take a 231 and go play with it? Talk about fun! That little grease gun wire stock, and no flash-hider made for a real Hollywood encounter :)