Single-shot guns are many, and they are often very accurate. The most practical single-shot designs are often more affordable than repeating firearms.
Rifles, shotguns, and pistols have all been made in single-shot versions. Single-shot actions may be of bolt-action or break-open design, but they are certainly not limited to this. They may also be based on falling block, rolling block, trapdoor, Martini, or other actions.
While many extol the virtues of hunting with a single-shot firearm, I'm not one of them. I was raised and trained by a very practical man who believed in carrying the most effective legal tool when hunting, and a single-shot just doesn't fit that description, except of course during muzzleloader seasons.

