What are considered to be the smoothest pumping shotguns out there?
The best I've felt are some of the early Browning BPSs. Not sure about their newer ones.
I believe you are correct. My ithaca 37 is a 1950 model and when it gets dirty it jams halfway through pumping and is a bear to clear.Smoother is not always better. The smooth action means a finer (tighter) fit. This means it is more suceptable to fouling because of dirt and grit. Your mileage may vary.