I'm not an expert.
What are you going to use it for?
Single barrel shotguns include single shot, pump action and semi-auto action. The other option is a double barrel.
I'm thinking you might be referring to a single shot.
Unless for a kid I'd suggest a .20 guage. While the .410 is a good round, it is exensive to practice with and options for it's use are somewhat limited. When using low-brass ammo, recoil is very low.
HR and Rossi both make decent single shots in relitively the same price range.