Effect gas blowing back through the hole depends a lot on the fit, hole to pin. Standard modification old Single Shot rifles is to bush the pin hole and fit up a new pin close. Effect of a blown primer with smokeless powder in Trapdoors, Rolling Blocks, other falling block actions with large firing pins and loose fit in the block can be dramatic.
Shotguns operating at lower pressure probably less, but loose fits lots of gas can cause real problems.
William
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