View Full Version : Question on gas gun operation.

March 4, 2012, 01:43 PM
Not specific to any one type, but in general. I've always wondered - what exactly are the mechanics that cause the force of the piston/op-rod/what-have-you to open the bolt and not the simple force of the recoil on the cartridge itself?

It would seem to me that the recoil of the cartridge firing would act on the cartridge before the gas in the barrel had a chance to operate the gas system. You are basically dealing with a backwards force in both cases. So why doesn't the bolt start opening directly from the pure force of the recoil? What makes the backwards energy delivered by the piston so different?


March 4, 2012, 01:51 PM
The gas gun has bolt and receiver lugs that the bolt rotates into place to engage, same as on a bolt rifle. If the bolt lugs are turned closed, recoil of the cartridge won't be able to open the bolt any more than it causes a bolt action rife to pop open when you fire it. Same reason. The bolt lugs transmit the force straight back to the receiver lugs which are tough enough to oppose it. When the bullet actually passes the gas port drilled into the barrel, that's when gas pressure starts to build up to enable the mechanism to rotate the bolt to unlock those lugs and then push it rearward. But it has to do things in that order. Bottom line, these are locked rotating bolt actions when fired, and the mechanism simply does what your hand does on a bolt rifle. Rotates them out of lock and pushes them back. These guns have springs that are compressed when this happens, and the spring then pushes the bolt forward to pick up the next round, closes the bolt behind it then rotates the bolt back into lock.

March 4, 2012, 03:38 PM
I see. How does the force on the piston get translated into rotational energy to turn the bolt?

March 4, 2012, 03:52 PM
It depends on the specific design, but most of them work through an intermediate body like a bolt carrier, that, as it moves rearwards, also causes a cam pin to rotate through a curved cam, or causes locking arms to retract into the bolt body.

Willie Sutton
March 4, 2012, 04:01 PM
And not all rotate, some tip up or down into recesses in the receiver. But all are locked until the bullet passes the gas port in the barrel.

M1 Garand animations here:


FN FAL animation here. The action starts when the bullet passes the gas port which is located at the front site. Gas then pushes the piston back, the piston whacks the bolt carrier back, and when the bolt carrier starts rearwards the bolt tips out of it's locking recess and then moves back itself.




March 4, 2012, 04:33 PM
Ok, that makes sense. I guess in hindsight I should have expected something like this. My simplistic view just sort of assumed everything happened in one huge shove. But the Garand video illustrates that it's slightly more complex - there's a alight initial backwards travel by the op-rod, which eventually hits a little widget that causes the bolt to rotate. Once rotated, the backwards motion that actually causes extraction begins.

March 4, 2012, 07:02 PM
The shove principle comes into play with blow-back auto-loaders, usually used for .22 rimfires - but occassionally used for smaller CF's too.

Blowback's bolts are unlocked, and the cycling is usually controlled via varying the strength of the recoil spring(s) and weight of the bolt/assy.


March 4, 2012, 10:20 PM
What you're describing is straight blowback operation. There is also delayed blowback operation, as in the 1911, which starts the cycle locked, but unlocks as the barrel pivots down. The principle allows blowback operation of a gun whose cartridges are strong enough that straight blowback would require a very massive bolt or heavy springs. The M3 Grease Gun and the British Sten guns, for example, are straight blowback, but they aren't designed to be holstered sidearms, where the weight and size would be unacceptable.

The tilt-down bolt mechanism is also described in Hatcher's Notebook if you want to see pictures of it.


Note the Garand op rod has a little hump just ahead of the handle. If you are looking at the hump from the side of the gun and could see through it, you'd see a slanted channel is milled into the side against the gun that slopes upward and forward. When the piston pushes the rod back, it drives the sloped side of that channel into a small extension on the bolt that sticks into it, wedging the bolt upward. That's what turns and unlocks it. After it unlocks, the extension is at the top of the channel and can't escape, so the bolt is then pushed backward into counter-battery by it. The slight initial movement you saw is just the slop in the fit of the bolt extension in the channel, which gives the op rod a little run-up to smack the bolt extension upward. In the M14 that extension on the bolt has a roller bearing attached to it that rides in the channel to reduce friction and speed up full auto operation.

James K
March 4, 2012, 10:31 PM
Some basics are needed here.

Some guns do work with just the pressure pushing against the inside bottom of the cartridge, which in turn pushes on the bolt causing it to open. Those are called "blowback" operated guns. The bolt is held closed until the pressure drops only by its own mass and inertia. (Think of the bolt as you in your easy chair watching a football game when the wife says, "Honey could you do a little thing for me...." THAT is mass and inertia.)

But when the cartridge becomes more powerful, the bolt mass and weight and spring just are not enough to keep the action closed until the pressure drops. If the bolt opens too soon, the case bursts, hot gas and pieces of brass and burning powder go all over, including the shooter's face, and that is not good at all.

So the name of the game is to keep the bolt closed until the pressure drops. Sure, you could make the bolt very heavy, and that would work to a point (Hi Point does just that) but if the bolt had to weight 35 or 40 pounds, not many people would like to carry the rifle around.

So the designers figure a way to keep the bolt shut for that fraction of a second. Recoil operation is one way, allowing the bolt and barrel to recoil locked together until something in the receiver allows them to separate. Gas operation is another, using gas tapped off the barrel to drive an operating rod which will in turn open the bolt. The AR-15 and its numerous clones uses a system called gas impingement, which does not use a piston or operating rod, but allows the gas to hit the bolt carrier directly; the carrier then opens the bolt and pulls it to the rear.

In most designs, once the bolt is to the rear, a spring will push it forward and reload the chamber.