RE: "Double bang"
it sounds like your gettting a hang fire for some reason. I have never shot the 209 primers, but my understanding is they throw more fire then the #11's. All I could think to check is make sure there is no oil or residue in the rifle before loading, and pick the nipple good before you fire. If it keeps up, change back to the way it was and see if it corrects the problem.... good luck