Might be a counter board to fix a damaged crown. I'm assuming the un rifled section at the muzzle It's slightly larger than bore diameter
That's what I was thinking until I reread what OP stated. It seems as though they're saying that it appears to be a smoothbore up until near the muzzle, in which you can begin to see rifling. I imagine it's similar to a rifled choke in a smoothbore shotgun barrel.
OP, is what you're seeing similar to these photos?
If so, I believe that there's only two realistic possibilities,
1) The rifling is absolutely chock-full of lead, which generally leads to inaccuracy. (likely)
2) The rifling is nearly worn out. (very unlikely)
It's probably number one, and you can clean it using a copper or brass bore brush. But if it still shoots well, I wouldn't worry about it.
It's most likely not number two because, while technically possible, it's nearly unheard of to wear out a .22 rimfire barrel simply through shooting it. Not to mention that if the rifle is in as good of shape cosmetically as you say it is, it likely wasn't fired
anywhere close enough to cause
any kind of perceptible wear to the rifling. About the only way I can think that your barrel could be worn out is if somehow your barrel missed the heat treating process at the factory and is therefore unusually soft. But that's
extremely unlikely.
If what you're seeing is similar to these photos below, then the rifle has simply been counterbored and is no cause for concern if the rifle shoots well. Counterboring is most often performed when the very muzzle end of rifling is damaged or worn by abrasion through the use of a cleaning rod made of steel. You see it more often on surplus rifles than commercial rifles, I've found.
