If ou call any supplier they will know what size nipple your gun will use. Hunting Dawg has given you the measurement. Don't ever hesitate to go to your nearest auto repair center and ask them if they have metric thread gauges. Hell, you probably can find them for sale at the Auto Zone, or parts stores near you.
YOu don't need musket caps in your gun. Take an empty gun out into your back yard, or dark garage some evening, and fire off a percussion cap. You will easily see a flame coming out the muzzle at least 10 inches long, with standard( not magnum) caps. Musket caps are only needed, sometimes, to ignite substitute powders like Triple 7, which have a much higher ignition temperature than BP. Because members have had varied experiences using substitutes, or " subs ", depending on the lot purchased, and how long it takes them to use up a can of powder, subs are not recommended often on this site. Some people report find accuracy, and good SDV( Standard Deviation in Velocity- the shot to shot difference for a series of 5 or more shots using the same powder charges and components) using the subs. But most others do not get good accuracy with the subs, no matter what it is. Pyrodex has been around a long time, and it is much better now than it was 30 years ago. It also seems to have a longer shelf life, than 777 or other newer subs. So, Please Use Black Powder. If you can't find it locally, it can be ordered from suppliers you can find here in the Member Resources section you will find in the top box of the index page. Try both FFg, and FFFg powder in that .54. With that fast rate of twist, you may find 3Fg powder works best with your conicals, but 2Fg powder works better with round ball. Or, your gun may just prefer the powders the other way around. That is the fun part of shooting black powder rifles. They are all individuals, and you have to find out what they like. Know that your gun has been designed with a very fast rate of twist to use with conicals, and was not designed with RB in mind. I hope your shoulder is up to a steady diet of conicals.
The problem you are having may be as simple as the hammer face not striking the top of the nipple squarely, so that the edge of the nipple gets peened down on one side after a few shots, and you begin to have misfires. Put some lipstick, or cold cream, or grease on to top of the nipple, and then lower the hammer to transfer the substance to the face of the hammer. Cock the hammer back and take a look. If the transfer is not an even width ring that is a complete circle, your hammer is not hitting the nipple square. Use a new nipple, BTW, for this, so that you grind away the high spots on the face of the hammer to make it strike the nipple square from the first shot to the last.
You will need to buy a small diameter grinding bit usually available at hardware stores for Dremel Tools. You can used a handdrill and don't have to buy a Dremel tool, although they are nice additions to your workshop. Grind away the face of the hammer where the lipstick appear after each trial. put a new, even layer of lipstick on the nipple, and try the hammer on it again. If you still don't have a full circle, keep grinding away the part where the grease does touch. That is the high spot. When you get to the point that you have a full ring on the face of the hammer, take some paper towel, fold it over, and put it over the nipple, cock the hammer, and release the hammer to fall normally. It should make a punch-like circle on the toweling, and may even cut through part of the bottom layer of the toweling. Take a close look at the paper toweling. The width of the impression of the top edge of the cloth should be the same all around the circle. If it isn't, you have some more grinding to do on that hammer face, but you are close to being done.
Standard #11 caps should do all the igniting you need using Black Powder. If you insist on using subs, then go to the magnum caps. Musket caps should be used in larger caliber guns for which they were designed, and not on a .54, IMHO.
