What I can say is Chain fire issue starts at .451 (and likely below but .451 is a good point to call for remedy)
FWIW, I shoot some .451 just to see if there is an accuracy gain. When I do I used a lubed felt wad.
I had done some testing with felt wads ahead of the bullets, removed the wad from the chamber that indexed up for the next shot.
You could see the blast marks on the wad, but it did not go much below the surface. In my world we would call that a flame arrestor (fuel tanks and flooded cell batteries).
My take was if anything did work its way around the ball (checked the seal, it was there, not as wide as .454 of course). The Wad would snuff anything flame wise. No issues so far though the Wad goes under the bullet.
I suspect it more like .448 that you would see no seal, but .451 is a good spot to take precautions.
the only sure way to ensure a seal band is to put a ball into each chamber and then pop it out with a punch via removed the Cone.
