I've been toying with the idea of a .44 mag. "Whisper" using a 400 gr. +/- boolit seated out but need a fast twist to stabilize that heavy a boolit. I think 1:20" would do it. The boolit would need to be either a nose/bore rider or the throat lengthened to accept the long boolit but it should work well I think. Dr. Richard gun designed a heavy subsonic boolit for .45 Colt/.45/70 that I would like to try in a scaled down version for this.
In the end if it worked the gun would be usable with standard .44 mag. ammunition as well but would be like shooting .22 shorts in a .22 L.R. chamber. The boolit seated out would allow max powder capacity for supersonic loading with heavy boolits too.
Just a thought. I like heavy and slow.
Longbow