I saw the following article and asked the author the same question, here is his response.
http://www.hpmuzzleloading.com/Technical6.html

Quote:
"With the current plug configuration and small flash channel,Blackhorn 209 DOES NOT perform very well out of the Knight KRB7 (Rolling Block). The hang fires are horrible...with the BP that Knight ships with the rifle.

But, as I stated, I've been "playing around" with it in a KRB 7. I took one of the plugs to a machinist friend, and had him open up the flash channel to .036". And ignition has been just fine. Later this summer, I hope to get a plug made that has all the internal and frontal dimensions of the standard Knight plug for the DISC Extreme models...only made to the length (and non plastic jacket configuration) for use in the KRB 7.

A concern Knight had with the KRB and Revolution was the backward flow of fouling into the action. But, that was with Triple Seven, which has a much faster peak pressure curve. BH209 has a longer curve, and lower peak preasure. I don't think the backward flow of fouling will be as great as with Triple Seven.

I shoot 100 and 110 grain charges of BH209 in front of the modified plug mentioned, and there does not seem to be any more fouling in the receiver than when I use the standard KRB 7breech plugwith FFFg Triple Seven."