The thickness of BP shotgun barrels are often as thin as the barrels of modern shotguns because the highest pressures accumulate more in the breech and not as much near the muzzle.
Some makers use a type of seamless tubing for smoothbore barrels while Pedersoli probably uses machined barrels since they also make double rifles that have a similar design to their double shotguns.

I really wonder if what you feel are a type of lathe marks from the machining & finishing of the bores. I've seen those circular, machine made lathe marks in the bores of their double barreled Howdah smooth bore pistols. I was surprised to see those marks rather than a really shiny mirror polished bore. But I didn't think that you would normally feel them when running a patch down the bore. But rather would be something that you could see near the muzzle crown, close up and with plenty of light.
I think that those circular lathe marks are normal and may indicate that the bore of this older shotgun isn't chrome lined like the later models. I have no way of knowing that for sure but it's just my guess. But companies sometimes do change how they make the same model if it's produced over many years and they might have added the chrome lined feature for waterfowl hunting with non-toxic shot.
The relatively lower price of this shotgun tends to indicate that it probably is an older gun since the newer Pedersoli's have become a lot more expensive. And if the barrels have fixed chokes then what you might be seeing/feeling are lathe marks and the price of the gun matches it's age and features.