docbill, A possible explanation. I've been told it was common among skeet shooters to use chamber inserts in there guns.The inserts ended just beyond the forceing cones.If the gun wasn't cleaned properly carbon rings would form in the barrels were the insert ended. The carbon rings acted as an obstructions causeing bulges directly behind the rings.Browning actually made these inserts for Browning guns making the bulges more common in Brownings than in other guns.
Dave