the buckhorn, knight wolverine, LK93, white rifles and like, use Plunger systems. You just pull straight back on the little cocking lever and thats it. The pressure that blackhorn209 creates, can spit the primer back into your face due to the plunger style systems not being strong enough. It states right on the bottle to use ONLY with an enclosed 209 primer pocket. The rifles i listed above use a slotted plug with a wire retainer. That wire alone can cause some nasty injury.
It has nothing to do with what the rifle is rated for. The breech plug design is what matters.
Correct Breech plug ( primer pocket)
Incorrect slotted breech plug On the LEFT
, ( Mostly found on the not recommended Plunger style muzzle loaders) Correct plug on the Right.