What is the thought of the 18.5 barrel with cylinder bore vs 20 inch barrel with improved cylinder.
Not really for in home defense but more for if shit was to hit the fan. I was thinking maybe a 20 inch with a full choke used on a 12ga. Mossberg 500. Gun now has a 28 inch barrel.

Shells used would be a wide range from 00 to #1 buck to #4 bird shot, all would be 2 3/4 and 3"

I've used both 20" defensive shotguns and 18.5" defensive shotguns. Either can work. But while the 1.5" doesn't seem like a ton of difference, I actually can tell a difference, and prefer the balance of an 18.5" shotgun.

That's primarily for closer range defensive purposes, though. For a SHTF "do all" shotgun, a 20" with rifle sights could be a better all-around option than an 18.5" barrel with a bead sight, to be able to reach out further with slugs a little easier.

Also note that you'll have 1 less shell in an extended magazine that's flush with the 18.5" barrel compared to a 20" barrel, if that matters to you. (Though it's entirely possible to use an 18.5" mag extension on a 20" barrel, or a 20" mag extension on an 18.5" barrel, if you don't mind the cosmetic offset.)

If it's a Remington 870, keep in mind that replacement barrels are tougher to find nowadays compared to years ago. The shutdown and bankruptcy of Remington, combined with the pandemic, put a serious cramp on barrel production and supply. And just when the new Remington Arms was starting to get up to speed on production, they're now in the process of shutting down their factory and moving production to another state.

I know when I was looking for 18.5" 870 and 1187 barrels a year or two back, factory new barrels were unobtanium. That may or may not have improved since. But if you don't already have the barrel, you might have to either seek out a used Remington 18"/20" barrel, or go with one of the aftermarket replacements like Mossberg (yes, they make 870 barrels) or Carlson.

IMO for Defense you want the 18.5 inch barrel with a Cylinder bore because in most older homes the hallways may only be 36 inches wide. In a confined space like that that 1.5 inches really does matter. The downside is that you will lose some velocity but in typical homes you won't be shooting at distances over 25 feet and you'll have more than enough retained velocity.

As for your 20 inch barrel with a full choke, this is a compromise from start to finish and NOT a good compromise. Shot shells today are produce to get peak velocities with barrels of 26 inches or longer. Basically you'll end up dumping a lot of unburned powder out the end of the barrel and you'll be down on velocity. There is a reason why a 28 inch barrel is the gold standard for a Hunting shotgun, it's because it's the best balance between weight, gun balance, muzzle velocity, and Point Ability.

Yes, maybe used or cut to length, then likely not threaded for changeable chokes.

I have a Benelli shotgun I wanted with a shorter barrel, mostly for hunting. I often have a 1” extended choke on it, giving another inch of barrel length.

For some threads, one can get 2” extended tubes. I just mention this in case someone wants to try to mix & match a little.

For all around i prefer a 20" with IC or modified choke. For HD I prefer 18".