Barrels are made out of steel and shot is made out of lead. It is a pretty easy fix.
While its true, the "fouling" you get, if you get any, won't be lead, it will be plastic. With modern shotshells, and their plastic shot cups and wad columns, the lead shot never touches the barrel. Nor does the "steel" shot if you are using that. (there is an issue with "Steel" shot and older guns with tight chokes, but that's a different thing.
Plastic can build up in the bore, if the rifling is actually digging into the wad column (sometimes it barely does, tolerances vary, you'll need to see what your gun and the shells you use, do. Removing the plastic isn't complicated but it can be a chore as its not "lifted" by the usual solvents.
Next point is the patterns. Rifling causes the shot column to spin and that spin spreads the shot pattern, and often to the point of significant size holes with no shot in them. Again, its something you have to test with your gun, your ammo and at different ranges. The "dreaded donut" is well known to those who shoot shot from pistols (with rifled barrels) and the "hole" isn't always dead center. Once you know how your gun behaves at what range, its not difficult to adjust your aim so that the "hole" misses and the shot hits.
It can be very different at different ranges, though generally the further away, the bigger the hole(s) in the pattern. And different size shot will be a bit different, and its not impossible different brand shotshells will produce differing results.
Test what you've got, and make adjustments, accordingly.
