Ok, so I just put a new 120 spring in the gearbox of my FAL. It was my first work on a gearbox...ever. I was careful to the point of paranoia, trying to keep everything exactly as I found it, except the spring. New spring (predictably) made the piston/piston body lurch out of place, and mess up my whole plan. I'm pretty mechanically inclined, so I got it all back together and it works great. Safe, semi, and full auto are all perfect. It will even pierce one side of a coke can at long last. Objective achieved.

But....

I've got two shims and a dinky, super-tiny, allen head, black, machine screw, left over on my table top. I've absolutely no idea where they came from, or what they were for. My mr.fixit obsessive compulsive disorder is driving me nuts. Any ideas what I did wrong, and should I risk re-opening up the healthy patient to find out?