I believe the biggest potential problem comes from using a wad for a tapered hull in a straight wall hull. I have read that this allows powder to migrate past the wad seal along with a poor seal upon ignition resulting in squib loads. However I have been shooting the newer Federal hulls from the 100 rd boxes sold at Walmart (straight wall hull) using the Claybuster 3/4 ounce wad designed for AA hulls (tapered hull wad). That wad is needed to get a proper crimp with 7/8 oz loads because the of the greater internal capacity of the Fed hull. I have not had any issues with that combination whether skeet shooting or hunting. Your mileage may vary.

On the other hand when using the BP1044 wad in a Federal 10ga (straight wall) hull I would get quite a few squibs. The 1044 wad is undersize and does not provide a good gas seal. The solution has been to put a thin nitro card or plastic gas seal under the wad.

BTW I found you can get excellent results by placing a 12ga wad on top of the BP x10x plastic gas seal. The trick is using a 12ga wad that gives the proper stack height for a good crimp. Patterns were excellent.

Thanks, I think. I guess the bottom line is "It's hard to blow up a good shotgun"

I doubt very much that anything I mentioned above would create an over pressure load. Substituting a CB 3/4 wad in recipe calling for a CB 7/8 wad for a 7/8 load wont change pressure, they are same wad with the exception of nub in the bottom of the shot cup of the 3/4 wad to reduce capacity. Adding a .070 nitro card beneath the wad in low pressure 10ga load isn't going to do much either. A 12 ga wad on top of the x10x if anything would likely reduce the amount of bore scrub which creates pressure. When starting out with a load that is 6700 psi there is room to be safe with that substitution. We are loading shotguns not rifles.

Yup.