For me its, the M9. Not because its inaccurate or anything like that but because I have nothing but bad luck with them.
The first time I fired one was in 1992 while a guest of the United States Army at Ft Lostinthewoods MO. The weapon was pretty new (not packing grease new, but new for the Army) and in 100 rounds I had at least a dozen feed failures. It reached the point where I was doing the fire and rack just to make sure it went bang when I pulled the trigger.
The next time I used the M9 was during training at Ft Bragg NC in 2007. The pistol was obviously not new, and likely spent many years in the training environment being maintained by Joe under somewhat sketchy circumstances. Not only did the failure to feed issue crop up, but I had several soft strikes on the primer (the firing pin hit with enough force to mark the primer but not with enough force to fire it). I'm guessing these were failure to seat the round properly due to excessive wear and tear...
In 2008, the M9 let me down twice while mobilizing for Iraq at Ft (sucks)Dix, NJ. While shooting pistol qualification the weapon locked up (slide wouldn't move, magazine wouldn't eject, trigger wouldn't move) after the first round was fired. According to the small arms armorer I followed up with the issue was repaired by the judicious application of C4 and det cord on the ordnance range.
The second issue happened after the third shot on the transition fire range (transitioning from M4 to M9) when the front slide went flying down range. Seems the disassembly lever release pin sheared off...
So yeah, it gets my vote for worst shooting because everytime I've shot it, its had problems.
