20 gauge vs 410

[DeanMk]

True. It came from a type of weapon known as a Garden Gun.
It was something small, light and unobtrusive to use when walking The Royal Gardens in search of "vermin".
I think the old H&R .44 cal smoothbore was a Garden Gun, too.

 

[BigGame]

if .410 was a gauge it would be 67 gauge.

This is right. Another way to think of it is that the .410 is to the 28 gauge what the 28 gauge is to the 12 gauge. The .410 is a lot smaller.