Yep,you're right, the proof mark won't tell you the wall thickness, you'll know if it's been re chambered or been honed to near death though.
We have our hands tied over here. It's not only bad practice but more importantly, against the law to offer for sale a gun that is out of proof. Barrels are often measured many times before a sale. The buyer may want an independent gunsmith to measure them for peace of mind so yes, it is important.
The point I was aiming at with the original question was even if the old barrels prove strong enough, could the action suffer from the repeated use of nitro ?
