What is a 410 shot gun?
This is a shotgun with an internal barrel diameter (called the
"bore) of approximately .410" which shoots a shell which is about
.410" in diameter.
It is the only shotgun shell size that uses this convention.
A 12 gauge shotgun, for example, does not have a bore of
"12".
A 20 gauge shotgun has a smaller bore than a 12 gauge and a 10
gauge has a bigger bore than a 12.
Gauge size is based on the number of lead balls of the bore
diameter which are required to make a pound (16 ounces). 10 balls,
12 balls, or 20 balls.