Denny, I have a DISC Elite which I think is the exact same action as the Extreme. I have both breach plugs (the one that uses DISCs and the one that uses just the 209 primer). For both, I don't think there is much of any fouling from the propellent, rather I think it may be from the primer. The breachplug that uses just the 209 primers is very clean. The DISCs have some primer fouling, but not that much. I find that I like using the DISC because they are easier for me to handle. I have a plastic tube that I fill with primed DISCs, other tubes with pre-measured powder charges, and others with sabboted bullets. I really like the rifle and would recommend it to anyone.