arcticap

There's room for differing opinions about wear of a muzzle loader barrel and what can cause it.
I tried to do some research but most [not all] is about breech loading guns.
But it does seem that projectiles can act like a lap in conjunction with the powder residue of which only 50% is actually burned and the leftover ash which is mostly carbon can be an abrasive.
There's also muzzle wear from loading just as there's ram rod abuse, depending on the ramrod material and the use of a muzzle protector.
If as Sam Fadela says that some barrels need a couple of hundred shots to break in, then one would think that they can also wear out over time.
I have a 2nd hand 50-60 year old muzzle loader with plenty of noticeable barrel wear.

There's heat, gas cutting, abrasive ingredients and by-products of the powder, the hardness and abrasiveness of the lead, alloy or other bullet ingredients and material.
There's a scale of factors that can cause more or less wear, although some would basically ignore them as not being major wear factors altogether.
It was mentioned that antimony in lead alloy can have the form of sharp, crystallized cutting edges embedded within the lead.

While some folks consider match grade barrel accuracy to degrade more quickly, others believe that they remain serviceable as hunting rifles for a much longer period of time, lifetimes in some cases.
Much less than a lifetime for others.
But that doesn't mean that they don't eventually lose their match grade accuracy and many muzzle loader shooters do notice it too, but perhaps they're not all inlines made from the top shelf steel either.

Another factor is that some inlines have shallow rifling that may not be best suited for the same conicals that were originally designed for the deeper groove rifling of traditional sidelock muzzle loaders.
Just like why paper patching is more suitable for some guns and not others.
It's not just the wear factor, but the best match of the bullet or type of projectile for the gun.

I did find one thread here where bronko22000 posted about barrel erosion and the use of heavy powder charges [See Post #24] --->>> Muzzleloader Barrels, care, breaking in and life spans

Everything needs to be understood in the context of to what degree do some factors create more or less wear on a bore than others.
They can be scaled in a relative sense, but it's very difficult to quantify all of them with any precision as to how long any barrel will last.
For most it's a non-issue, but for some others it might be.
In the end it's still all about which style of projectile does the shooter prefers for their gun.
I have less of an issue shooting a traditional style conical through a traditional sidelock than with an inline.
But usually not too many due to my personal preference and shooting results.