I agree with Don.

Barrels that are simply back for a reproof should not exhibit this.

I can not imagine a powder or charge common to small arms that would still be building pressure 26+ inches from the chamber. The only thing that comes to mind would be a howitzer.

We have been instrumenting barrels for nearly 120 years. We know full well how things behave. Too much black powder and it gets blown out unburned. Too much smokeless or too fast a burn rate and you rupture the chamber.

Are they testing with some alternative (none lead) shot in an attempt to lower the over airborne lead at the proof house?

Pete

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