WADR, the caps sold today are already too hot. There is no need to use a musket cap to ignite your Black Powder. If you are using substitutes, you deserve the problems they create! You might as well buy an adaptor to fire modern pistol primers, as you are trying to ignite smokeless powder, by any other name!
If you can fire both musket caps, and regular percussion caps out of the same gun, do so at night, or have someone fire off a couple of caps for you in the dark, so you can see the length of the flame coming out of the barrel. I have not found a gun that had a long enough barrel that I didn't get fire coming out the muzzle at least 10 inches. The musket caps will do that and only a little more. Musket caps are used in the larger bored guns, and rifles, because they put out a larger spray of fire, which helps ignite more of the main charge. But even percussion caps put out enough heat to light anything.
If you are having ignition problems, its because either the nipple or flashchannel is blocked. The length of the flash channel is of little concern. That primer explosion will send out a flame that is more than capable of traveling a few more millimeters to reach your charge, even going around corners. But it can't do that if the flashchannel or nipple is blocked.
When I am reloading a percussion gun, I use a damp patch down the barrel to move the crud, but I am also watching the nipple to see that some smoke comes out of it. That proves to me that the flash channel and nipple are clear for the next shot. I then run a dry patch down to dry the barrel, before loading my next powder charge. This process requires me to pull the hammer back to half cock, after firing, and clear the expired cap from the nipple, before cleaning the barrel. I found that when using the #10 caps, I had to use pliers to remove them from the nipples. #11 worked just fine, split, and were easy to remove. After loading the gun, the last thing I do before capping is to use a cleaning patch to wipe the nipple clean, and clean the smoke and crud from around the nipple, and hammer. That gives me an excuse to check the face of the hammer, and skirt for damage, or stuck caps, so I can eliminate those possible problems before re-capping my gun to fire the next round. It also cleans the sides of the nipple so that spent caps don't stick to it.