Kynoch

Have you thought about having the culprit re-stocked, a nice Weatherby California style sloping comb should look nice atop your double

The root cause of the phenomonen you describe was often caused by drop from nose of comb to heel, as most vintage double rifles were made expressly for use with iron express sights so the stocks were made with lots of drop, this tends to accentuate recoil and allow the muzzles to rise, far more so that the nmore modern assymetric stocks (high parallel comb) which are popular where scope use is envisaged, in the latter, recoil impulse tends to be straight back, hence more felt recoil on the shoulder but less muzzle climb and hence less chance of the comb battering the lower jaw bone.

Such things as changing the recoil pad are unlikely to solve this particular problem although recoil reducers in the stock may be of assistance.

JT