Let's talk about the FAL trigger! Specifically the hammer, trigger and sear.
The trigger and the sear are on the same axle. The sear has an oblong hole though.
With the rifle cocked, the nose of the sear is engaged in a notch on the back of the hammer. The trigger return spring and the hammer trying to rotate cause the tail of the sear to sit just above a shelf on the top of the trigger.
When you pull the trigger, it pushes up on the tail of the sear and that causes the nose to pivot down and disengage from the notch in the hammer.
BANG as the hammer drops and hits the firing pin.
Without the pressure from the hammer shoving it back, the sear is pushed forward by its own spring and the tail drops into a step below the shelf on the trigger.
When the bolt carrier comes to the rear and cocks the hammer, it is caught by the nose of the sear.
The trigger is still pulled at this point.
When you release the trigger it lets the tail of the sear come out of the step and rest just above the shelf.
That's how semi-auto works.
All military FALs had what is called a safety sear. It sits forward of the hammer and is spring loaded. When the bolt carrier is not fully forward the rear of this sear will lay in a notch on the front of the hammer and prevent the gun from firing.
The shaft that's rotated by the selector has a groove in it. On SAFE the rear of the trigger is blocked by the shaft. On SEMI the tail can move up into the groove and move the sear off the hammer. On AUTO the groove is a little deeper and lets the trigger pivot far enough that the sear doesn't catch the hammer.
Without the safety sear the hammer would just follow the bolt carrier and in most cases the gun would stop firing.
What the safety sear does is hold the hammer until the bolt carrier is all the way forward and hits an arm that will pivot it out of the notch on the front side of the hammer. In auto it holds the hammer until the bolt is closed and releases it allowing the gun to fire again until the trigger is released and the normal sear can engage the rear notch on the hammer.
It was quickly discovered that a full-auto FAL is not very controllable, so most nations disabled the auto position. Most replaced the selector with one that lacks the deeper groove for the trigger. Many metric pattern guns also got a small pin sticking out of the side of the trigger housing and a hook that limits the travel to between semi and safe.
On inch pattern guns the selector has the deeper groove, but a "wing" or tab was added to the selector that would contact the upper receiver and thus prevent the auto position from being engaged. I think that this was done so the troops could bend the wing out of the way and use full auto in an emergency, but that's pure speculation.
On almost all commercial guns there is no safety sear. A very few semi-auto FALs were imported by Browning and have an ATF waiver and are called "sear cut" guns. Rare and expensive.
What selector you have on a commercial gun depends a lot on where the parts came from. Many parts kits have the auto capable selector and trigger housing. In this case you should use a pistol grip that blocks the selector from going past the auto position.
OK. That covers the trigger. What about forward?
Let's start with a chambered round.
When the hammer falls it hits the rear of the firing pin. That sends the pin forward into the primer and sets off the cartridge. The resultant gas pushes the bullet down the barrel and past the gas port, letting gas into the gas block and to the gas plug.
The plug has two settings. Open and Grenade. When open it lets gas into the gas tube and when on grenade it blocks the flow of gas completely. I have variously heard that this is either to prevent damage to the rifle since the gas is higher pressure for grenade blanks or that a rifle grenade needs all the gas it can get to launch properly.
If the plug was in the open position the gas can begin to push the gas piston to the rear. Right around this point the vent in the gas regulator is exposed and some gas is allowed to escape. A threaded collar allows the user to adjust how much gas is vented. This allows you to "tune" the gun to function very smoothly and to add more gas to keep things running once they get gunked up.
Gas Plug in Open Position
Gas Plug in "Grenade" or Closed Position
Gas Regulator and Vent
The piston pushes back against the face of the bolt carrier, moving it to the rear. A spring inside the gas tube returns the piston to its rest position. This compresses the recoil spring and lifts the rear of the bolt out of the locking shoulder, extracting the empty casing, knocking it out of the gun and then when the recoil spring pushes the carrier back forward; strips a fresh round out of the magazine and pushing the rear of the bolt back down against the locking shoulder allowing the cycle to continue.
Rear of the Piston
The worn circle on the bolt carrier is where the piston strikes to move it to the rear.
The normal FAL has the recoil spring in the buttstock and the bolt carrier has a "rat tail" or rod that engages a follower on the end of that spring. The rod is pushed into the stock behind the spring when the bolt carrier comes back.
"Rat Tail" with the Bolt Carrier Forward
"Rat Tail" with the Bolt Carrier Locked to the rear.
"Rat Tail" engaged with the Return Spring's Follower or Plunger
On a PARA FAL the spring is inside the top of the bolt carrier and compresses against the rear face of the trigger housing.
I hope you enjoyed this little tutorial!