I can't find it so

[Bruce Day]

Calvin, people have generally said here .025 for a 12ga , but the British proofing houses will pass down to .020.

I am personally uneasy with anything close to .025 in a 12. Many light small bores came from the factory at close to .025, so that is a bit different. Most Parker 12's were factory produced at least .035. I would have to look up the hoop stress formulae again, but the greater the diameter, the greater the wall thickness needs to be to withstand the same internal pressure.

I know you are a 10ga guy and I don't have enough experience with the 10's to know the average factory wall thickness on them or what would be acceptable after honing. I know lots of 10 ga guys shoot some heavy loads. If your intent is to load 1 1/4 3dre, then that is such a light 10 load, it wouldn't make much difference. If you want to experiment and see how much you can get, you need to know more than I can tell you.

[Wild Skies]

Originally Posted by Bruce Day

..... the British proofing houses will pass down to .020.

I don't believe that is a totally accurate statement Bruce---and may be a bit misleading. In direct correspondance with Michael Lynch, the chief inpector at The Birmingham Proof House. He told me that they (the BPH) will inform their customers if the MWT falls below .020 even though it may have passed proof below that number.