DeltaBoy, 2piper is right (as usual). Relatively "minor" bore obstructions (and I hate to imply ANY obstruction is not very serious) will cause bulges. As increasingly severe obstructions are tested, they create increasingly larger bulges, then ruptures. Obstruction ruptures usually are local blow-outs rather than splits.

Greener in "The Gun" illustrates bulges created by light wad obstructions, pictured next to a blow-out caused by two light wads with just a small fraction of shot between them.

Fred