Joe Wood Its not particularly modern proof, I think that barrels that only have a very small contact area of rib to barrel can be opened up by the much larger pressure pulse moving up the barrels. Alternatively, it may be that there is a bleed hole there already but it is blocked by rosin, varnish, loctite (one commonly used bodge by barrel blackers), rust or baby food, who known! A good bang and the hole clears to produce a delicate stream of bubbles on the next test.
I remember a bit of high speed film of a load moving up a shotgun barrel and the bulge moving up the tube was quite visible. I have to admit it gave me pause for thought when I first saw it!
