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Whew, it's getting deep in here!
First, the mechanism shown in the diagram is the semi-auto rifle, the only ones sold on the market here, AFAIK. The sear mechanism is exactly the same as that on the M1 carbine. When the hammer is cocked, it sits against the sear, forcing it back against its spring. When the trigger is pulled, the rear of the trigger pushes up on the rear of the sear, moving its front down and out of engagement with the hammer. The hammer falls, firing the rifle. The bolt comes back and pushes the hammer down. Meantime, the sear has reset itself (the disconnect function) and catches the hammer, which again forces the sear back. When the trigger is released, it moves down so the sear can ride up over the rear of the trigger and wait for the next trigger pull. This system has been used since the Winchester autoloaders of the early 20th century. Some things can go wrong with it, but not many.
Second, the pictures by Slamfire show what FN calls the auxiliary sear, (auto sear). It is not present in the rifles sold here. It works like this. When the selector is set for FA fire, the standard sear is disengaged, so the hammer, if not stopped, will fall and follow the bolt down. But the hammer is caught at the front (not at the top like the M16) by the auto sear, which holds the hammer back until the bolt carrier goes forward all the way and the bolt is locked. Then the shoulder on the bolt carrier trips the auto sear, disengaging it from the hammer and letting the hammer fall. This continues until the trigger is released, when the regular sear again takes over and keeps the hammer back. It is not a "safety sear" and has nothing to do with safety; it is an auto sear, pure and simple, and is not used or needed in semi-auto rifles.
Third, the trigger spring seat (as good a name as any, I don't know the actual name) is used on wood handguards. It does not appear to be needed on plastic handguards as they are strong enough to support the spring without the seat.
HTH
Jim
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