Haven't seen a #1/#3 as a regular muzzleloader, but I have seen a very fine #1 built into a breechseating muzzleloader, complete with false muzzle. In typical fashion for this type gun, it seats the charged case in the regular fashion, and then the bullet is loaded into the muzzle and seated against the case.
A friend owns this gun, and shoots schuetzen with it, with very respectable 200 yd. groups. His gun is fitted with a custom .32-40 barrel.