At a local gunshop that I frequent, a very nice Browning o/u came (can't remember the ga.) in with both barrels bulged just in front of the hand guard. We all speculated about the cause and it eventually came out.
The fellow had been using sub guage chamber inserts that left a powder fouling build up/ridge at the point of the bulges. This fellow didn't clean the bore before he went back to shooting regular ga. shells and the barrels bulged. The general consensus was that he scrap the gun.

W. E. Boyd