Stay away from that adapter, it is extremely dangerous! H&R made a muzzleoader from their single barrel break open shotgun back in the 70s. There were 2 versions, one had a threaded breech plug and the other had a press in plug retained by an O ring. It's possible for the adapter to blow out if the breech is opened after a hangfire and the gun goes off. Allegedly a shooter was killed not long after they put the gun on the market when that happened. The breech plug caught him in the head. It would have been 45 or so years ago and I can't find any documentation currently though it was widely reported at the time and H&R stopped production of that variation and it was the only gun we ever banned from our shoot.