The early long range target shooters in some countries didnt trust like the new fangled fixed cartridges and continued to load the early cartridge single shots from the muzzle. A later version of this was in the shutzen matches where the bullet was breech seated into the rifling and a charged case behind it.
I have seen HR single shots converted to muzzle loaders. Some were fairly crude with just a breech plug fitted and the chamber left in the back, but if you used enough powder to get to or ahead of it they shot decent. Others were done as a stub barrel and very nice.
Dine right a bolt action with extractor and ejector altered for the 209 primers would be interesting.