A pre '64 Winchester 94 in .30-30. No matter what I did, or what loads i tried, or handloads I tailored, that thing wouldn't keep rounds on a pie plate at 100 yds off a bench and sandbags. Shot my Brother-in-laws Marlin 336 right next to it, and was pissed at how much better it grouped.
I've been a 336 fan ever since. Though I've had friends with fine shooting Winchester 94s, I can't bring myself to buy another.
My Olympic Arms AR15. It was my first AR and I learned a good chunck of what I klnow about ARs from that thing breaking and failing so often. Always had a tight mag well, so mags never just dropped free. The upper to lower fit was extremly tight, to the point that i had to drive the take down pin out with a mallet and a delrin punch. The bolt broke in under 500 rounds. The hammer pin cracked. Teh gas tube retaining pin walked out and left the gas tube floating and made the rifle a singe shot for a while. The gas key bolts started walking out, causing a gas key leak and more feeding issues. The front sight pins were so tight that it took a torch to heat them up to be able to drive them out once I go around to just scraping and parting the rifle out, and the front sight base turns out to be a cast part instead of a forged one. The barrel was the one redeeming quality, becausee it was always accurate. I still have the barrel, and that is the only original part I still have of that rifle.