Someone correct me if I am wrong on the exact number, but a .410 is a .68 gauge shotgun. I remember this back from my hunters education course. So when people say: 10ga. , 12ga. , 28ga. , etc. the .410 would be called a .68 gauge.A rifle has a rifled bore, a shotgun generally does not. Although a "sabot" slug is made to be fired through a rifled barrel, it's still shotgun ammo.
A .410 bore (gauge) is a shotgun. It's a small shotgun.