That's like asking how much does a car weigh. London Best game guns tend to have thin walls by American standards - maybe 28 thou or so.

I've never measured a Birmingham boxlock NE barrel but I bet they're thicker.

Minimum is what passes proof. 20 thou is considered thin; I have been told that 18thou barrels have passed proof. Dunno for sure. I think there used to be a legal minimum but again dunno for sure.