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Using the wrong wad can result in sub-par performance if it results in “powder migration” past the seal, or if significant “blow-by” (gas escaping around a load rather than pushing with all its force behind it) reduces load pressure and velocity. So variances in gas-sealing ability and the different pressures these hulls thereby produce are one reason never to substitute wads.

Most modern wads are designed to “obturate.” This means that the bell-shaped gas seal flares out slightly under pressure to create a self-sustaining seal. Engineers who design shotcups understand that a tight, consistent seal gives your load maximum drive and consistent ballistic performance.

High velocity loads require a tight gas seal in the hull and in the shotgun’s bore to achieve quality results. The modern phenomenon of back-boring or over-boring barrels, enlarging the bore diameter slightly beyond SAAMI standards, has become popular without much consideration of the procedure’s effect on the shotcup’s ability to seal the gas behind the load of shot. Of course, we are only talking about a few thousandths of an inch, a virtually impossible difference for the unaided human eye to distinguish, but it is significant in a manufacturing and shooting environment where the difference between success and failure is often no more than that. If gas escapes around the wad seal in an overbored barrel, velocity and pattern will suffer.