So if I bought one of these guns, could I also buy a longer vent rib barrel and swap it on if I wanted to take it out for some sporting clays? Or is the body of the gun set up to only accept that heavier barrel? Has mossberg done anything goofy with their magazine tubes like Remington recently that makes their field and HD guns non-interchangeable on the barrel front?
Yes. The 590A1 with a 5-shell tube (like I linked to in my previous post) uses Mossberg 835 barrels. Do not use 3.5" shells. The perfect 835 barrel for what you're asking about is 90831 (28" VR, parkerized, ported, front and mid beads, screw-in chokes).
https://www.mossberg.com/cgi-bin/co...tes\BW-Center\SearchResult.html&category=B835
A guy here did it. Looks great.
http://www.mossbergowners.com/viewtopic.php?f=22&t=1203