One thing I would like to add is BH209 doesn't really like a skirted projectiles like Powerbelts. They do not seal tight enough to adequately ignite the power....IMHO
I've tried them many times and the inconsistency was a dead ringer for me at least, besides even BH209's sight talks about compression and the seal needing to be good.
"A muzzleloader is different than a cartridge gun, but many of the principles relative to the propellant are the same. The bullet is seated in the cartridge to create the pressure necessary to propel the bullet. A bullet in a muzzleloader needs to be compressed against the powder. The base of the bullet, or sabot needs to expand to hold the building pressure which propels the bullet. Imagine you placed a bullet 1 inch ahead of the case in a cartridge gun. The bullet most likely would not exit the barrel. This same principle works in a muzzleloader. Loose fitting bullets, like the typical Powerbelts, may be convenient to load, but lack sufficient compression to assure consistent ignition or accuracy.
Because there are no standards in muzzleloading barrels, the diameters vary between all manufacturers and at times within the same manufacturer. A Powerbelt may fit nicely in one and slide to the bottom of another. Just the simple act of carrying your gun in the field may allow the bullet to slide forward and when compression of the powder is lost the result is a misfire or poor accuracy.
We recommend a quality tight fitting sabot for the most consistent accuracy and ignition."http://www.blackhorn209.com/specs/ignition-guidelines/With that being said I would first change out to a sabot projectile and I would get a tight fitting sabot at that. Now if the OP is loading in normal temp, then takes it out to a extreme low temp I would suggest that the light plastic skirt is getting very hard and therefore there is a possibility that the skirt is not sealing properly and/or it is not being able to keep consistent compression on the powder. Either way I not use Powerbelts with BH209.
Now I don't know what your barrel is sized at being a 54 cal, and if I were you I would take it to a good machinist and have it measured, and go from that measurement to a sabot that will seat correctly. With more fascia on the bullet/sabot pushing out to the barrel instead of just a skirted Powerbelt, well I think it will solve your issues.
Also my gun likes the CCI 209M primer better over the Federal 209A. Just an idea.
Also use a bare 209 breech plug.
My 50 cal Knight (Green Mountain Barrel) likes oversized .452 bullets with looser 50 cal Harvester Black Crush Rib sabots. Maybe try something similar?
