Well I thought I would add a bit this evening.
My 20ga damascus Flues gun has a thin spot of .018 @ 16" from the breach in the left barrel. I was really apprehensive about shooting it after this discovery, even with low pressure shells.
So, I figured what the heck, I would blow it up or shoot it. Loaded 4ea. hand loads @ 12,500psi per the IMR manual and did the string/proof/behind the tree test. I measured the barrels at a bunch of points before and after.
All is good.
Carry on. ymmv
First off let's establish a few facts here. At 16" from the breech those barrels can really care less what the max chamber pressure is. What they are concerned about is what the pressure is @ 16" from the breech. "IF" you take two loads of the same intensity, IE, same shot load fired to same velocity but with widely varied max chamber pressure. It is a Given that forward of the chamber area for the rest of the length of the barrel the load giving the "Low" pressure load will have higher pressures than will the low pressure load.
To "Protect" a thin area in the forward 2/3-3/4 portion of the barrel keep your shot load light & velocities moderate. Then load the fastest burning powder that will meet the criteria of desired max chamber pressure commensurate for the barrel mat'l.
Assuming that your "Proof" load @ 12,500 psi was done with at least an ounce of shot & used a slow burn powder, then you should be in fine shape with 3/4oz or no more than 7/8oz of shot with a faster powder up to about 9K psi.
IF you load 7/8-1oz of shot to a bit more velocity & lower your max pressure by going to a very slow powder you are not doing that thin spot @ 16" any favor.
