RE: CVA Mountain Stalker
First off, the breech plug does not come out of that rifle. If you try to remove it you will damage the rifle. My Mountain Stalker Manual (I have a .54 caliber) says your rifle has a 1:48 twist. There was a real problem when they were selling these rifles, they advertised them as a 1:32 but in reality they were 1:48 twist. Perhaps you got a special one. You can check the twist with a tight fitting patch on a jag by pulling it up and counting how much of a revolution it makes in a certain distance.
The Mountain Stalker weights 5.5 pounds so if you load it hot, it will kick like the devil. You can soften the recoil by taking the plastic butt plate off. Inside you will see a hollow stock. Push a rag down into the hollow to the bottom. I then took some #4 buck shot and vacuum sealed it in a plastic bag. I then sealed it again, and form fitted it to the hollow. I pushed that in the stock and put another rag on top of the buckshot to keep it in place and replaced the plastic butt plate. I added about a pound and it made all the difference in the world for recoil.
The max charge for that rifle is 100 grains of FFg according to the manual. If you load 100 grains of powder I do not think you are going to want to shoot more then that. In my rifle I shoot 80 grains of powder with a wonderwad between a patched roundball. The accuracy is exceptional. If your rifle truly is a 1:32 twist, then I would suggest some pure lead conical bullets. CVA Buckslayer sold through Cabela's should shoot good with about 80 grains of FFg. Start them out at 70 grains and work your way up 5 grains at a time. Also since your sighting in the rifle swab between shots. Since this is a sidelock, I swab with pure 91% isopropyl alcohol. It helps stop the hangfires and misfires. I run a wet patch and then two dry ones.
Another one I would try is REAL conicals. In .50 caliber the reals should shoot real well. I would use a wonderwad between the powder charge and the conical. I think they are a 280 & 320 grain conical. I would have to check my molds to be certain (I make my own for both the 50 & 54). REALs shoot excellent out of my Stalker, but the best is still the roundball.
You might want to try some .490 roundball with a .015-.018 patch. Start them out at 60 grains and work your way up. Do not adjust the sight until your sure you found the load you like. Your rifle might really shoot them well.
The idea of the hot shot nipple is an excellent one. I never keep the factory nipple in any of my rifles... Also if you e-mail CVA they will send you a manual for the Bobcat/Stalker. Actually they send you a manual for the CVA Sidelock Rifle Warranty Information. You used to be able to down load their manual off the web but since they stopped making the sidelocks I will not say that is the case... I also changed out those sights for a cheap set of STAR DOT FIBER OPTICS SIGHTS that I bought at Wal Mart for about $22.00. They really made a difference. Although I deer hunted with my Stalker that first fall I got it and shot three deer with it. The furthest being 40-45 yards and three deer never knew what hit them. I was shooting roundball.....
Good luck with your rifle.