Not all barrels are the same. I have one in the shop now with a wall thickness of .040" and others as thick as .080" before the choke. That's a huge difference. Additionally the barrels are made from DOM tubing, which makes for large thickness variations around the OD of the tube. So, if you had a nominal wall of .040", you'll have some places where the tube is only about .035" before the choke. Shooting a slug through a full choke barrel like that is quite a bit different then driving it through a .080" thick barrel.
Tony