In all my ML 's it the primers that gives me the blow by not the powder . I fired 15 primers ( no powder ) through a clean BP and it got so fowled up with carbon, i had flames coming out the side . It didn't do it with 1-10 primers shot . As the carbon builds up in the breech plug the hole down the middle gets smaller and smaller , then i got tons of blow by . I gave up using 209 primers , IMO they are to hot for some BP's and BH209 needs a hot primer . so i have gone to using 777 powder and a .25acp conversion that uses small rifle and pistol primers . With this conversion i get NO blow by ever because for one, i get a 100 % seal and two , i use weaker primers that do not load up the breech plug with carbon . It's a win win for me . I was replacing breech plugs every tub of powder and the Bh209 is more $$$ , they want 38.00 compared to 24.00 for 777 . here is a photo on my Encore BP after firing 15 primers only ( no powder ) from a clean breech plug . I used a drill bit to clean out the flash hole and fire channel before doing this test .

I shot a 1/2 tub of Bh209 with this new BP ( photo below) and you can see the flame cutting caused by the Win 209 primers going across the ring in the bottom of the primer pocket . Even with the longest 209 primers the carbon would build up and the blow by would get very bad and even start blowing the primer apart and bulging the side of the primers .

http://i365.photobucket.com/albums/o...ckDSCN0154.jpg

same breech plug with only 5 primers shot through it and you can see it starting to get blow by and i really started to get bad at around 10 and i stopped at 15 . I also got flame/fire cutting from the blow by in the primer pocket of the BP ands once that starts , you will no longer have a good seal and you can kiss that BP good by .

http://i365.photobucket.com/albums/o...tsDSCN0163.jpg

NO more problem since i started using this BP .http://www.thebuckskinner.com/breech_plug.html as you form fit each brass to your gun by closing the breech on the brass giving you that 100% seal and a spotless gun , also my gun got so bad the firing pin got stuck in a primer and i could not open my encore and i had to remove the hinge pin to get it apart , then remove the firing pin and cleaned it removing the carbon from it . that was the final straw and is stopped using 209 primers and any powder that needed them . It was a good move for me !!