Pete, just to illustrate my point: this burst was caused by a mix up in the powders used to reload the shell.
The loader inadvertently used smokeless powder in an amount prescribed for a black powder substitute (could it be called PB?).
There was no obstruction.

This was posted here in 2006 together with the injury the poster suffered on his arm. You'll probably remember.


JC

"...it is always advisable to perceive clearly our ignorance."ť Charles Darwin