funny gun law and like allways in such a situation people looking for a clever interpreting. my first thought was its made for the civil war breech loading carbines to make them like muzzle loader by law.
this is the right thing for the german 40 mm long lancaster bullet cartridges especially if you work with the Gras carbine. many of them were sporeterized as cheap shotguns in 20,24,28 and 32 ga. it would be possible to rifle the shotgun barrel making the gun barrel diameter the land diameter and shot the 40mm long brass case with lead bullets in this.
if the gras action is still original the 32 ga brass is the right stuff and fit the 11 mm gras bolt head. btw, .54 muzzle loader bullets fit like a glove in 32ga magtech brass.

50 Smith brass and plastik case,50 Maynard,54 Gallager, 54 Sharps( one of the different cases design popular for replica sharps in europe)

56 Spencer CF,56 Spencer CF(different case) 32/40 with 54 cal Muzzle loader bullet, 28/40 for my Husqvarna 17a, 20/40, 17mm Danish Snider RF, 18mm French Tabatiere