Plastic build all depends on how smooth or polished your bores are. Plastic is not really a big problem and is easy to remove . When using card and fiber wads without a shot cup you will definitely get lead build up which is harder to remove than plastic , but still not a big problem .

As many routines as there are shooters. Here's mine.

I wipe the bore out with some Balistol on a blue shop towel right after shooting. Pay attention to the Chamber, bigger sometimes the patch won't get it like it does the barrel. I have a separate short rod just for chambers. Outside wipe down with Balistol too. If using the gun again soon that’s all.

My modern O/U using cheap promo loads with soft shot seems to fowl more than reloads with quality shot in the Parkers. Or it could be the Chrome plated bores show it more. Plastic causes no extra problems. But I don't use Plastic of any kind in Black Powder loads. Like Mark said fiber wads let the barrel lead more than a plastic shot cup.

Every few times I use the gun will clean more thoroughly. Have a couple of Delrin rods threaded one end for a brush other end has a power screwdriver bit inserted so I can Spin it under power. You could do the same thing wiping, under power it’s faster. Depending on the bore may use JB Bore Paste or 4/0 Steel wool lubed with Balistol. If they are rough little extra help speeds the job.

I wipe out the inside of the frame with a towel & Q tip too. Re-lubing very sparingly. Other thing is I always pick the gun up day after and give it another quick wipe before storing.

William

Aerosol Brake Cleaner

Plastic build up is more of a problem with guns that have replaceable chokes. My wife's O/U guns get it all the time, but my old SxS's don't seem to have the same problem.

Don't have any plastic problems with Parkers either. However am using quality components. No end to the length manufacturers will go to save a fraction of one cent per shell. No doubt promo loads use the cheapest wad, and they may build up plastic in bores.

Something builds up in my O/U may be lead may be plastic. It’s on a steady diet of cheap factory loads.

William