It has to be a #11 or Musket cap, and it has to be opened to the elements. That means no 209 primers and no Knight Disc rifles or others that shield off the nipple from the elements. I read it somewhere last season when I went muzzleloader for deer and elk. I thought it was in the regs under muzzleloader equipment, but I could be wrong. If all else fails, call Olympia and talk to somebody in the Enforcement office. I forget the guy's name that I always ask for, but they would be the ones to ask, NOT the license or general info number. They don't know their butts from a hole in the ground when it comes to game regs. And get the person's name when you call. Another good place to call for muzzleloader info is Muzzleloader Supply down on River Rd in Puyallup. That is their business, so he's definitely up on the rules.