Years ago, there was a guy here who went by the board name of Greg Tag. He kept track of Flues failures. IIRC, most or maybe all of them were 20's, they were very light, and at least in some cases the owners had fired "light" 2 3/4" factory loads through them. Often inexpensive stuff from big box stores. While they may have been light in terms of shot charge, they weren't necessarily light in terms of pressure. He'd pretty much identified that as a common factor in quite a few of those 20ga Flues guns. The frame typically cracked right where the standing breech meets the water table.