I've used muzzleloaders for years without a problem, but heard that the nipples can fail catastrophically, causing injury to the face. Is there a way to tell when it's time to replace them?
I've only been shooting traditional muzzleloader for the last few years, so I'm not an old hand with them. But I would suspect that you'd begin to see cracks around the top edge. I frequently remove mine and make sure it's clean inside and out. Honestly, I've never heard of that level of failure on one.
I hunt and shoot with guys that have used them for decades and I don't ever recall ever seeing them even replace the nipples. If there's an ML/BP club in your area, it might be worth it to look them up and ask around there. I'll check in with the gentlemen who is helping me build a rifle currently and ask him about it.