I use a plastic coated springy steel rod in the .36, made by Kleenbore, I think, no damage. It is used for cleaning and loading when shooting at the range.
use a 5/16" stainless rod for cleaning and sometimes when range shooting the .50 Beck. No muzzle protector, no damage - yet. Maybe only 1,000 rounds with that. I use a 3/8" drill-rod for loading and cleaning me .69 with the rifles usage rod used only on trains. No discernable damage no muzzle protector. Likely used for loading only 3,000 times though but used for cleaning EVERY time since 1986. Go figure! It's my Match rifle.