Rimfire
A cartridge family where the primer compound is spread inside the rim of the case rather than seated in a centered cup. .22 LR is the most common example.
Rimfires fire when the firing pin crushes the rim against the chamber edge, igniting the priming compound spread around the rim’s interior. They cannot be reloaded because the rim is consumed.
Rimfire training is the cheapest precision practice you can get. A .22 LR ladder at 100 yards teaches wind reading and trigger control at a fraction of the cost of running centerfire. Damnosus’s RLR thesis applies hard here. A precision .22 setup at its useful range tests the same skills as a magnum at 1,500 yards.