Thanks for all the info. My question came largely from thinking about Damascus's go / no-go bore gauge. I was thinking about how that would affect the decision about a guns safety.

What I'm understanding is that if a gun is well out of proof, there's reason to suspect the walls might be overly thin. A gun being in proof is no gaurantee of good wall thickness but is a better bet.

Here's what I found on my Reilly, using a Manson wall thickness gauge and a skeets bore gauge:

Right: 0.655" diameter and 0.068" wall thickness @ 9" from breech. MWT 0.048" @ 9" from muzzle

Left: 0.657" diameter and 0.070" wall thickness @ 9" from the breech. WT 0.048 @ 9" from the muzzle and MWT 0.044 @ 6" from muzzle.

From the flats, I see the gun was proofed at 17/1 bore, which is 0.655". So the gun is well within proof (suggesting it hasn't been excessively honed, etc) and has plenty of wall thickness throughout to be safe to shoot.

[[Linked Image from i.ibb.co]

Jim