400:

Can you elaborate here? When you say multiple, are you saying they are placed loosely in the case and can shift about or something? What exactly are the offending wads?

BTW: I don't own a double, but do shoot a load I worked up for a Marlin .45-70 using a wad column and a charge of IMR3031 followed by a card wad + greased felt wad + card wad and finally the bullet. This affair provides for compression of the powder and very clean burning, not to mention almost total elimination of leading even in cold temps using the Lee 400 {402 ave cast of wheelweights} grain Hollow Point bullet. Previous loading using just the powder allowed unburnt powder and leading of the slightly undersize bullet {.458 in a .458-to-.459-inch groove-depth barrel}. I have no clue what pressures are generated by my pet load but it runs completely safely in my gun.

I "shake test" old rounds to make sure no airspace exists before shooting.

I am wondering if a long wad column would be safe/useful in the straight {+/-} .450 BPE using a stacked deal like the one here, albeit thicker due to longer case length.

Incidently, my .45-70 load here runs 1640 fps and with it I've killed deer, bear and range cattle. The bullet provides very deep penetration "Nosler Partition-style" performance.