Traditional muzzleloaders are as reliable as the operator. There is a long and continuous learning curve one must master to get reliable and accurate shooting from a traditional rifle. You seem to be looking for a crutch to get by without doing the work to make it right. Those devices to use rifle and shotgun primers instead of the correct percussion cap have been on the market for at least 50 years that I know of but I have never seen one on a rifle because they really aren't needed if one will just clean the rifle properly and load it properly with genuine Black Powder. Caps must fit the nipple snugly but not so tight as to be difficult to seat them fully. If the cap is too tight the first hammer fall just seats it and it will need another drop to fire it. If the cap fires but the powder does not the problem is almost certainly in the flash channel, the small hole which connects the nipple to the powder chamber. Because it is of small diameter it is easily blocked by fouling. That must be cleaned thoroughly, run a pipe cleaner through there every time you clean, which should be as soon as possible after the gun has been fired. Clean with plain water, dry it out completely and don't get carried away with the oil, a little is plenty, you should not be using so much oil that it actually runs out if placed muzzle down on a paper.
The "substitute" powders, like Pyrodex, add additional problems. The very reason those powders were produced is because they are harder to ignite than black powder. In theory that makes them safer to ship and to store but it also makes them harder to ignite in the gun. Still, one can do OK with Pyrodex if they will just properly clean the gun, paying attention to the flash channel.
It's always better to "fix the problem" than to go to some backward extreme to try to force it to work despite the problem. Ask any knowledgeable shooter when he last had a misfire with his traditional sidelock percussion rifle and he'll probably say "so long ago I can't remember".
