Most CVA and Rossi muzzleloader rifles take breech plugs that are 5/8" x 15 threads per inch. The easiest way to make a breech plug to fit these rifles is to start with a Grade 8 bolt that can be purchased for a couple of dollars online. I have found them on Ebay and at Home Depot. If you are a competent machinist, a neater job can be done starting with 1" stock and cutting your own threads. I start my project by running my bolt through a 5/8 x 15 die. I've found by trial and error that bolts generally are a couple of thousandths oversized and they turn into the threads of my rifle more easily if I do this. These dies can be purchased on Ebay.