Joel/Calgary said:
Like Paul (and Dave?), I have NEVER had a charge shift or an O/S card move with 16ga components in my 16ga double (happens to be the proper fit), even with repeated firing of the same barrel as a test.
I've used double guns a fair amount and never found a shifting load in the second barrel to be a problem. I never check it, because I'm not afraid of it.
I think people make far too big a thing out of the possibility because it doesn't work they way they imagine. It seems that people believe a shifted charge creates an obstructed bore and the danger of a burst barrel. I don't think that is the case. By far the heaviest part of the charge is the shot, and only the overshot card is holding it in place. If any shifting takes place it's only the pellets and the overshot card which move, not the overpowder wad. You then have the shot in a larger space, and it won't stay in a compact mass, it will lie loose on the bottom of the bore, with space above it. If you fire the gun in that condition, it just blows the overshot card out of the way and a lot of the gasses escape. You will know it has happened, because you get a big whoosh, low recoil and a strange, hollow, reverberating sound. I had that happen once, and it was obvious what was going on.
In his book The art of Shooting Flying, 1767, Thomas Page described the problem and the sensation you get when it happens perfectly:
"After some experience you will find, if your gun is clean, and the wad thrust but lightly down, that in walking the shot will be apt to get loose: and if you discharge the piece in that state, it will seem, by the small resistance it makes as if there were no shot in it:"
Spence