Believe it or not, a M2 .50 cal that belonged to the Michigan National Guard.
I spent 11 months with my .50 cal, ran like a swiss watch. My platoon was loaned to the 101st for about a month, away from the rest of the company. Had to leave my .50 behind and borrow one from a Michigan NG battalion HHC. It was the most beat up weapon I have ever seen, but I figured hey, its a .50 cal. Mine had been dropped of trucks a few times and blown up countless times and still worked. This one had spent the last year inside a conex so it couldn't be that bad.
Took it out to test fire before a mission... 2 shots then FTF. Checked headspace, timing, gave it a bit of lube...2 shots, jam. This happened a few more times until the LT said, screw it, we have to go. Ran that mission hoping nothing happened I couldn't handle with an M4.
Took it to the 101st armorer... he couldn't find anything, said try a different barrel and recheck headspace and timing. Next day, new barrel, had headspace/timing checked and rechecked by 3 different ppl. Got to the test fire barrel on the way out the gate... 2 shots...nothing. Boy I was mad, had to run another mission with a 2 shot .50cal.
Took it back to the armorer, went over it with a fine tooth comb with him and the 101st 1SG, who was also an armorer. Couldn't find anything wrong. Armorer told me to recheck headspace and timing (grrr), 1SG said BS, the gunner is not an idiot, send it to the small arms repair shop on base. Comes back a day later, small arms shop can't find anything wrong, but replaced the springs and extractor anyway. That night, test fire lane... 2 shots... nothing. Yet another mission unarmed.
After that, thankfully our replacements had shown up, and we jacked one of their .50 cals while they were getting settled in. When I went to turn the POS M2 back in, the Michigan armorer (who was different from who issued me the weapon) took a look at the weapon and says "This one never was supposed to be issued out!" I guess he knew it was a junker too. Last I ever saw of that POS, never figured out what was wrong with it... good riddance.