I've worked on almost every gun I own.
Modifications vary. My only pistol in use right now is a full-house USPSA Limited gun built on a Glock 35. It has an aftermarket barrel, adjustable fiber-optic sights, 2# trigger, magwell, upgraded springs, etc, etc. It is very reliable, although two of my extended magazines need new springs.
My rifles are all AR-15 platform guns. All of them have match triggers. All have heavy-duty extractor springs. Two have optical sights - an EOTech 552 on my defensive carbine, and an ACOG TA11F on my 3Gun rifle. My HD gun has a float-tube foreend and a LaRue light. All my working magazines have been modified for function. I think I've had one failure to feed with my carbine, probably 1500 rounds ago.
My shotguns - well, I have a lot of shotguns. One of my Franchis is pretty much stock save for an extended magazine. The other has an extended magazine, ghost rings, sidesaddle, and a stock cut down to fit me better. I've never had a failure of any kind with either Franchi. My skeet gun is a Remington 1100 LT-20 with a trigger/action job, custom-built stock, and barrel bored out to Skeet I. I can't remember ever having a malfunction with it with light loads, although it will choke on the heavy stuff.
- Chris