Brent,
There is simple and ancient test for blowby in cartridge shotguns with card and fiber wads. Take the combined fiber and card wads (you are actually using) and shove them down the bores from chamber, using a cleaning rod. IF they are hard to shove down the bore (especially if very hard), they probably seal nicely. IF they slide down easily (especially if really easy), they are very likely not going to seal.
One can also do some pattern testing with the overpowder card and fiber cushion wad loaded over a plastic sealing wad -- cut from base of a one-peice plastic wad or use one of the separate plastic sealer wads. IF you were getting blown patterns with the card and fiber wads alone but, get much better patterns with the plastic sealer wad under them, you have your answer.
I am always leary of old doubles having the bores reamed to remove pitts. Can/will really ruin them for loads using card and fiber wads. I have two such old guns that give pretty useless thin and patchy patterns with card and fiber wads, but, good to excellent patterns with one-peice plastic wads.
How well card and fiber wads fit into a case is not the important criterion for blowby. Sometimes one can go to thin brass hulls and the oversized wads they require to improve patterns.
Niklas
