A dozen or so years ago a man named Oscar Gaddy was the guru for damascus barrels. Gaddy's rule of thumb was that a shotgun barrel, any barrel, should be proved for 20,000 pounds, that modern barrels should have service loads of no more than 10,000 pounds, old damascus barrels should not have more than 6,000 pounds, and that any shotgun barrel should not blow up until 30,000 pounds. This assumes that the chambers are left exactly as they came from the factory.
Gaddy experimented on an otherwise sound G grade damascus Lefever by reaming out the chamber from 2 5/8" to 3",,,which did not blow up the gun even with proof loads,,,,then by reaming out to 3 1/2" the Lefever failed on the first shot with a standard factory shell, which made no more than 14,000 psi. The same barrel passed 20,000 psi proof with a 3 inch chamber.
The problem with reaming out a half inch of chamber is that you are taking the metal out of the most critical part of the barrel that has the least reserve for higher pressure. The makers taper the barrel very quickly starting at the end of the factory chamber they cut in the barrel.
I have a long forcing cone reamer. I only use it to give long forcing cones to single barrel pumps and autos, that have barrel walls with lots of extra metal. I never, ever lengthen a chamber on anything.
Cutting one half inch extra in a three inch chambered barrel is asking for disaster. Don't do it.
