Well... if you are asking for DIY mods then that opens up a whole new can of worms! I thought you meant making parts, as in CNC etc. That is a major PITA and it makes much more sense to just buy them. As far as modding or DIY, that is done ALL the time, which is why dremels are a must have for techs. I constantly have to mod parts for various reasons, mostly due to minute manufacturing inconsistencies or incompatibilities. I will see if I can think of a few of them:
Trimmed the "fins" of a V3 spring guide to fit in a FAL because the proprietary spring guide is just a V3 with shorter fins, and it needed a metal ball bearing one.
Trimmed/removed countless teeth on pistons to correct AOE, remove second to last tooth not already removed, short stroke (12 teeth) and half stroke for DSG (8 teeth).
Removed teeth on sector gears for Angel short stroke pistons.
Removed material from safety catch due to inconsistent specs.
Swiss cheesed pistons
Unwound a coil of an AR latch spring to reuse a broken one.
Bent AR latch springs so they would work in other version gearboxes.
Taken motors apart and swapped axles to turn a long into a short and v.v.
Put longer screws in the handgrip where it meets the GB shell because only the lowest threads were stripped, thereby not having to buy a whole new shell.
Done the same as above with handgrips.
Bypassed the selector plate electrical connection used in most gearboxes so that I could use a normal wiring kit in VFC and G&Gs, etc. by bending the two leads to touch and soldering them together.
Made all kinds of funky "adapters" for electrical testing and battery usage purposes.
stretched cutoff lever spring for a crispier and more reliable semi auto
Dremeled out a screw in the top of a V2 because it had been totally stripped and therefore could not be unscrewed (the threads where not stripped, the top was). Three can hold it together just fine
Shimmed springs for a little FPS boost
Trimmed tappet plates for DSGs
Added Xrings and synthetic orings to piston heads, and teflon to cylinder heads and hopup chambers
Added a TM nub to the roller in a PTW hopup
Trimmed buckings to fit various other manufacturers
Stabilized barrels with homemade spacers
Custom cut barrels to fit, both outer and inner (See why I say a Dremel is so useful? LOL)
Transplanted trigger contacts (An M14 had burnt out contacts, like completely burnt out. He wanted the gun the next day and I didn't want him to have to buy a whole new wiring harness. So, I took a V2 one, cut the contacts out, laid it on top of the burnt one and soldered it on. Hence, a "transplant". Works fine to this day)
Trimmed spare screws to fit places they aren't supposed to go because the originals were missing.
Added spacers on hopup chambers to keep their setting better.
Widened the back of a Tavor shell because it was too tight for a piston.
Replacement hopup clamp mod as Stephen said, nub thing of course, etc.
Custom foamed suppressors
Custom foamed around GBs and in various other empty places where things reverberate.
Added more sorbothane to cylinder heads.
Rewired things in all sorts of funky configurations.
Someone had soldered a wire to a motor unnecessarily, so to save the next guy who had to work on it (likely me) from having to solder and unsolder every time it had to be opened, but knowing I couldn't clamp directly to the motor, I added a male and female clamp about an inch from the motor itself, so it could still be unplugged for disassembly.
Added shims between chambers and GBs to fix feeding issues.
Same for mags.
Trimmed a little off the part of the shell that keeps the contact slide from going too far back in order to install the highest toque gearset possible.
Widened the bearing holes in shells so they would fit.
Ummm... that's all I can think of at the moment I guess.
. If I think of anything else I will throw it in there.