Aftermarket breech plug! Also, apparently some guys have cut off the base of a hi-brass shell and primed it. Then they loaded the gun as a muzzleloader. I'm not in any way endorsing the practice...just wondering if anyone has tried it. Seems iffy to me, but there's guys on youtube that roll with it.
H&R Huntsman originally came out with a threaded breech plug. Came with a special wrench.
As a cost cutting move they replaced them with an O-Ring sealed breechplug. The actions were made so the breech would open enough to get them capped, they need to be disassembled to remove the plug. It was easy to break or modify so they opened like the shotguns. Some discharged when closing. One of these plugs caught the shooter in the forehead killing him. It was written up as a warning in MUZZLE BLASTS MAG (official NMLRA magizine) in the 1970's/80's.
I have seen the video where the guy "converts" his gun to muzzle loader by cutting off the shell and thought it was hilarious. Just get a Lee shot shell loader and roll some black powder shells.