I also recently purchased a .54 Lyman deer stalker. The stock nipple works fine with #11 cci brand caps. The only thing you wanna do when shooting it is just wipe the nipple off here and there with a dry patch. This is so that the primer continues to seat as far as it has to every time without a failure to fire occuring.
Another thing. While the cci #11,s do fit on the nipple well enough, it's not a completely snug and tight fit like a number ten remington cap on say an 1851 with stock nipples or another revolver with slix shot nipples. I've found that if you lower the hammer onto the cap slowly before actually going to full cock and firing, the cci #11 works every time. You will want to do this slowly and carefully though, or perhaps push it on with a small wooden dowel.
Maybe remington #11 or rws caps will be more snug. I personally love the rifle and have no complaints about it. It shot like a foot high out of the box, but zeroing it brought it center of mass. I wasn't really trying for accuracy that day, but 50 to 100 yards was no problem. Swab the barrel every couple of shots and clean up will be easy. I fired 48 rounds out of it that day and plan to try for 100 this weekend.
I use .530 hornady round ball and tried .015 and .018 patches. Both work but .18 is tighter. Enjoy your deerstalker.
One last thing... The rifle will be covered inside and out with a factory preservative that will take a while you clean up. The barrel will take some effort to clean. I went through a ton of patches trying to get all the factory oil out of it. I used a solvent, ballistol and dry patches.
