I put together BP clay loads for my 20g SxS and use the on the grouse set up.
Way too much fun.
I have found that an equal volume 21g load does not pattern as well as the square 24g load.
This is annoying because i can get a star crimp on the 21g but need to roll the 24g.
(I like to use a length of wad that exceeds the diameter of the bore).