As far as I know, the primary English makers only warrant or recommend bismuth as a non-toxic shot for their old shotguns, including those with Damascus barrels. Westley Richards, H&H, etc.
The warning about ITX in chokes speaks volumes as to it's propensity for flaring or enlarging muzzles with chokes.
I loaded a .487" ITX ball in a pistol barrel. It turned sideways, rocking on the trip down the tube- that was just loading it, not firing. The corners in contact with the lands, were peened over, showing the metal will move, however so did the corners of the lands. It was a GM barrel, which is harder than some ML barrels. I've warned the makers of this and they are going to try to reduce the size. The band on the RB's is the problem as corner to corner, it is larger than the bore by some .005".
Due to the speed of discharge, I am concerned - for the rifle barrel, and also for the choke. Of course, their warning about chokes should be adhered to.