OK, so can a “civvy” buy one of those 18(?) rd tube magazined Remington 12 guages?
Yep, just depends on where you’re located as to if they are avaliable.
not 18 round, the tube would be almost 4 feet long. The shortest barrel a “civvy” can buy is an 18” - 18-1/2”. With a full length tube that should hold six or seven 2-3/4” rounds plus one in the chamber.
I purchased a Rem 870 Express some years ago for $189. 30" barrell pump with a four shot mag (plug in the mag to make it two for hunting). Interchangeable choke tubes. I had the modified it came with for bird hunting and bought an extra full for turkeys.
A few years ago I bought an 18" barrel with rifle sights for it. Interchangeable chokes, it came with a cylinder bore choke for slugs and buckshot. I got a rifled choke tube for the same purpose. Remove the hunting plug the gun holds five rounds (one in the chamber). With that rifled choke it stays in 5" at 100 yds with the rifle sights. It also patterns buckshot well at 30 yds. Bonus, put the plug back in and the extra full choke and that short, sighted barrel is even better for turkeys. Your mileage may vary.