Depending on who made your barrel ,if its one of the aftermarket barrels offered today , it may in fact be made from the very same material as a center fire .
Traditionally however , muzzle loading barrels were made softer either by material used or purposefully by annealing them . On those types of barrels , even wood RR can produce wear on the muzzle do to dirt and grim being embedded in the rod . In fact many times on an original rifle , one can discern if the owner was left or right handed just by the wear to the muzzle of the soft iron barrels .
IMO fiberglass rods are some of the worst rods one can use as the glass can damage the muzzle even faster then a metal rod .
The plastic rods , act much like a wood rod in that dirt and grim can embed in them and then have an effect . But it takes time .
We are talking right at the edge of the lands . The rest of the rod but for the tip , will not touch the bore during loading or cleaning .. The bore guide helps save those edges at the muzzle
As to your powder load question .
Yes , if your shooting for string, the powder charge has an effect as does the lube , patch and even brand of powder . Some even suggest that the amount the powder is compressed also will effect the accuracy .
There is however no doubt that once you work a proper load for your rifle , there after the more consistent you are with what you do , the more accurate you will be.
IE take your cartridge reloading . Consistency is key in that you can have variances and still end up with a round that’s good for plinking or accurate enough for hunting. But if your target shooting for competition , you want ever step and component of the load to be as close to the same as possible .
Same thing with muzzle loading competition shooting
