I recall an article that reported one of the proof houses in the UK deliberately fired a Baikal 12 ga. barrel with a deliberately obstructed barrel by placing a 20 ga. shell ahead of the 12. With high speed videography of the barrel, the pressure bulge could be seen travelling down the barrel and out like a rabbit going down a diamondback with no apparent damage to the barrel. I've also read claims by Benelli doing the same with one of its pump guns.