[sc_rufctr]

"We" used to call it the breach block. It rides inside a "Slider" that has internal cams that engage and move the rear of the breach block up and down.
As the slider rides forward and chambers a round the cams lower the rear of the breach block into a locked position.

The round is picked up by the face of the breach block which has a claw that snaps onto the base of the round.
The hammer hits an anvil that is in contact with a spring loaded firing pin. We would normally never dissembled the breach block in the field.
As an added security measure we would carry the breach block with us whenever the rifle was stored or in the barracks. In theory if everyone had their breach block no rifle could be fired.

The breach blocks were unique to each rifle but they could be used in other rifles in a pinch although not 100% reliable.
The amount of gas that goes into the piston cylinder is adjustable.

They work very well and properly maintained they are very reliable. The biggest issues is the weight of the rifle. It's heavy compared to an M16.