Probably more like he wants a pistol chambered in the 45-70. If you have an 18" barrel is it considered a rifle and is illegal to be equiped with the pistol grip.

The cut off point where a long gun becomes a pistol and vice versa is 16 inches. Any barrel length over 16 inches is a long gun and any barrel length under is a hand gun. You can not equip a barrel greater than 16 inches with a pistol grip nor can you equip a barrel less than 16 inches with a shoulder stock.

For a shotgun it is 18 inches and there is no hand gun category for a shotgun. The barrel must be a minimum of 18 inches long regardless of what grip is installed.