Is your Ideal's rifling designed for slugs, or is it designed to produce an even more open pattern to a shot charge than cylinder bore? There are quite a few French doubles with the right barrel designed as a "canon raye dispersant". Designed in particular for woodcock shooting.
this barrel is not the short rifled section near the muzzle that is noted as "raye". this is the shallowly rifled full length rifling noted as "supra". follow this thread from
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=492115&page=7and i will add another observation; the english paradox system, and evidently the "raye" rifling is added inside the dimensions of the normal bore. it is effectively a whirled choke. this "supra" rifled bore is proofed at .716, and measures .716 on the lands; but has grooves 10-11 thou deeper than the bore. that fact seems significant to me.
