Around work, people come to me with broken firearms. I've installed aftermarket stocks, mounted scopes, replaced firing pins and various springs. On occassion, I've fabricated small parts - with my moto-tool, drills and files - to replace ones that I couldn't find anywhere. There's a machinist up the road who handles all of the harder stuff. He recently made a custom muzzle-brake for my MAK-90.
Everybody thinks I'm some kind of gun-whiz, but 95% of the stuff I do I think everyone else should be capable of doing. I like to tinker - sometimes extensively - with my guns, but functionality and safety are always priorities.
The attachment below is my old Marlin model 60. It sat in the closet for years with a busted stock until I decided to "repair" it. The only original parts are the reciever and barrel.