Blackhorn 209 requires a certain degree of resistance from the projectile in order to ignite the powder charge properly. It needs an environment of pressure in order to ignite. This pressure is provided by a full-strength primer working against a bullet that fits tight in the bore.
Both the Great Plains and Powerbelt bullets do not provide this resistance and they begin to move enough when the primer fires to reduce the chamber pressure and let the "fire" go out, so to speak.
BH209 should be used in a muzzle loading rifle only with saboted projectiles that are snug fits to the bore. I would bet your results will improve if you substitute a bullet and sabot that require a short-starter and at least moderate effort on the ramrod to fully seat the projectile.
I use BH209 in a Knight Disk rifle with a bare 209 conversion, Federal 209A or CCI 209M primers, a green Harvester Crush-Rib sabot and a .429 Hornady 300 gr XTP bullet with good results in cold weather.
Your comment about resistance makes some sense but doesn't exlain why I get two rounds off perfectly while the rifle is still at or close to room temperature.
The powerbelts do go in a lot easier than the Great Plains bullets do. I can get the powerbelts in with no started but the Great Plains bullets take some effort to fully seat.
It sounds like the bare primer conversion is the way to go but then I have to take my gloves off to load one which is no fun in the cold. Damned if you do damned if you don't.
