Ranked in ascending order of awfulness:
- .45 GAP — Glock’s answer to… what was the question again?
- any “Short Magnum” rifle cartridge — e.g. .300 WSM, .270 WSM, etc. They don’t do anything their longer predecessors can’t already do, their sharply-shouldered casings can cause feeding problems, and they were really just a sales gimmick like today’s new darling, the 6.5mm Creedmoor.
- FN 5.7x28mm — expensive and designed by modern-day Europeans, it’s basically a .22 Win Mag, just worse.
- .22 Hornet — while a good performer, it’s difficult to reload because of the tapered casing, and other .22 cartridges (e.g. the .223 and .222 Rem) perform as well or better.
- .25 ACP / 6.35mm Browning — as the saying goes, if you ever shoot someone with this little thing, and he finds out about it, you’re going to be in trouble.
Your suggestions on the topic in Comments.