You're right, it's IHMSA. As I've gotten older, I forget all sorts of things like an occasional letter. Did you know any of the early founders? Elgin Gates, Gary Riley, Jon Shaver, or Bert Stringfellow? The best I ever did was a state wide competition in California, at the Fresno Pistol and Rifle Club, shot AA with a three five seven. T/C., got 52 out of 60 shooting from the Creedmore position. I got a pat on the back.
Best shooting I ever did. You are correct, the turkeys were an S.O.B. to hit. Tall and narrow. I used to like to spot for other shooters and in the right weather conditions, you could follow the bullet all the way to the target. Really fun. Here in the Boise/Meridian/Nampa/Caldwell area in Idaho, out of all the sporting goods stores (Cabela's, Sportsman's Warehouse, Cliff's, Larry's, etc.) the only shop that had any percussion caps was Cliff's and those were CCI musket caps. I wish it was different. I notice most online outlets don't have anything either. You say the powder and caps are available. From who? I am registered at Midaway USA and MidSouth Shooter's Supply. I'm not familiar with others but I know they are out there.
I enlarged the hole in the musket nipple to exactly the same diameter as in the original #11 percussion cap nipple that came with the rifle. I wouldn't think that would be a problem. I am aware of the blowback, that's why I didn't take it out any larger. Well, if it wrecks the nipple, it wilI be a lesson learned. I really enjoy black powder and my Hawken, I never owned one before. Back in the early 80's, I put together a CVA Pennsylvania .45 caliber rifle and shot it for years. I had access to some really 'hot' percussion caps back then, 1981 to 1990, and no problem with any grade black powder. My son has it now and he actually goes squirrel hunting with it. It really aggrevates his buddies, he's a better shot with it than they are with their unmentionables. Right now we are enveloped by smoke from several forest fires, the AQI is 147, visibility is maybe 2 or 3 miles. It really sucks just to go outside.
Great talking to you and learning something new. Take care, have a great evening.