How (some) guns work.

Gas operation seems to be a confusing topic for some people.  I am going to try to make it better.

There are several types.  Piston, annular, direct impingement and some others.  What distinguishes them?

First, lets talk about some terms.  The "stuff" made by the powder igniting when you fire the gun is called "gas"; this is what pushes the bullet out of the barrel.  In the way-back days gun designers like John Browning (but not just him) noticed that a lot of gas was being wasted and they wondered if it could be harnessed to work the action of the gun.  It could be, and they did.

The Bolt is what locks into the breech and keeps this from being a bomb when you pull the trigger.  A bolt carrier is the part that moves and unlocks the bolt from the breech and lets this be a repeating process.  This is called "working the action".

Lets start with the most common type:

Piston: A piston gas system is almost the simplest in concept.  There's a port drilled into the barrel, this lets gas pass into a tube (called a cylinder) where there's a piston.  This piston is driven back (just like the bullet is going forward) to move the bolt carrier.  Springs push everything back forward, lather rinse repeat.  This is what the famous AK uses.

Annular: This is actually a piston system, and is the second gas system developed.  here the piston tube surrounds the barrel, and so does the piston.  So we have three concentric circles.  An outer casing for the gas tube, the piston and then the barrel.  It works the same as a piston system, it's just harder to make and maintain.  The German G.41(M) and G.41(W) used this system.

Lever: The very first gas system!  Here the gas port vents the gas against a lever that then works the action through linkages and camming.  It's not very efficient and has parts moving around outside the gun.  The Colt Potato Digger is an excellent example (perhaps the only example).

Direct Impingement: Here's where some confusion arrises.  Direct impingement is where the gas is bled into a small tube and fed back to strike the face of the bolt carrier directly.  It's not a popular way of doing things since it makes a mess in the feed area and the gas hitting the bolt carrier tends to erode it.  The Ljungman Ag m/42 and MAS-49 employ direct impingement.

Stoner: This is often called direct impingement, but it's not.  It is its own thing.  Gas is passed through a tube, like in direct impingement, but instead of striking the bolt carrier, it is ported into an expansion chamber inside the bolt carrier.  The expanding gas pushes forward against the bolt (which has nowhere to go) forcing the bolt carrier to move to the rear.  In effect, the bolt is the piston and the bolt carrier is the cylinder.  The huge advantage this system has is all of the reciprocating forces are directly in line with the barrel.  This lets the recoil forces move straight back instead of trying to rotate slightly.

There are also variations on the theme.  Very early Garands used an expansion chamber forward of the muzzle to capture enough gas to operate the piston.  In some piston guns, the piston and bolt carrier are the same part or the piston is attached to the bolt carrier permanently.  It can be very confusing!

In the Garand the  bolt carrier and piston are one part (called an operating rod or op-rod).  What makes this a piston system and not direct impingement is the end of the op-rod is in a cylinder and there's no small tube running gas back to the carrier.  The M14 (and M1A) is based on the Garand but a small, separate, piston is added near the end of the op-rod.

The Mini-14 has a bolt carrier that looks a great deal like a Garand, but is actually a variation on direct impingement.  Instead of a tube carrying the gas to the carrier, the end of the carrier is up against the port.

I hope that cleared up some confusion.