Another question I hope you muzzleloading veterans can answer: How do I properly protect the muzzle of the rifle to ensure accuracy?

I once owned a T/C Renegage in .54 cal and the barrel quit grouping consistently. Not sure why, as I've stated in another thread on this board. I am posting the question here because the failure of my Renegade caused me to give up the muzzleloading "hobby" for a few years, and I truly enjoyed shooting that rifle. I had eventually switched to blackpowder only and I only shot round balls I had made from my Lee mould. I routinely used "modern" lubricants to protect the rifle and the bore. Any ideas? I don't want that to happen to the Lyman flint rifle.