Prof, It does not really matter if a few bits of powder migrate as long as the shells are consistent in velocity and pressure. Some powders are better than others for migrating and the small ball or semi ball powders have always been know for migrating by the wad seal to some extent. It was a real problem with some .410 wads to the point that you had to increase the powder drop when using old empties. About 15 years ago I had several different hulls tested and found that after two reload I had to increase the powder drop by .7-1.2 grains to keep velocity up. It was caused by increased internal volume in the hull and greater powder migration with the orange PC wad being the worst, but still it gave the most consistent loads for velocity and pressure. After that I stopped worrying too about powder migration and decided velocity and pressures were more of a concern. Keep them as close to the same as possible and getting good patterns. I wondered at the time if the migrated powder did not still get fully burned up anyways and expect it mostly does. If the powder gets by the seal I expect a bit of fire does to burn the powder as well or as the wad exits the hull then the migrated powder should burn. So I load straight wall hulls with wads that may not prevent powder migration if they preform well in testing and patterning.
I loaded 5,000 Fiocchi 12 gauge hulls trimmed down to 67mm to shoot in both 2 1/2" and 2 3/4" guns. Loaded with Unique powder, load was 1 1/8 ounce, 1200 feet at 6400psi and a 1 1/16 ounce load which was slightly lower pressure. Loaded about 2,500 of each load. I had them tested, the loads that is, before I mass produced them on my Spolar loader. They were almost on the money, with extreme velocity variation of 35 fps in one and 16 in the other. Pressure varied more than velocity and the highest pressures did not relate to the highest velocity. The 1 1/16 is a really nice load that will even cycle a semi auto for my daughter in law, who shoots a Benelli SB3. In a SxS or pump it is a pleasure to shoot. I might just load only that load with 1 1/16 as it crimps nicely with out issues and seems to get the job done.
I no longer worry about what to shoot, if I want to shoot any 12 bore I have, from a 1860's Westley Richards hammer gun to modern gun. I just reach for a box of these Fiocchi and shoot without worry. They fit everything, are a safe moderate pressure and a decent velocity for any clay target. And I only load Fiocchis with these two low pressure loads, with common wads, a easily obtained powder and primers and used a standard crimp so no special loading issues. I have another 10,000 once fired hulls which I got for less than a penny each. When these 5,000 are shot up then I will load another 5,000. I even have 16 pounds of Unique I bought just before Alliant started increasing prices through the roof. Other than hunting loads in 12 gauge I am set for a year maybe in 12 gauge.
