Just bought one of these and had called Remington prior (so not like I was suprised). If you purchase one of these, the newer ones that include the extended magazine (like the 6+1) are "single" magazines and not technically an extension. If you procure one with the single (one-piece) magazines then you CANNOT swap out barrel's down the road. So if you get the 18.5 Tactical option with a single MAG it is what it is. Now if you want to swap barrel's to make it dual purpose then look for one that has the extension (2 piece). They stated that there are some still out there, but all new ones are coming with the single extension now.
Wanted to share.....
FYI on Rem 870 Tactical gun
Re: FYI on Rem 870 Tactical gun
Yeah the single tubes the way to go. If a guy wants dual purpose it would be best to start with the longer barrel and 4 round tube and get a second shorter barrel and extension. I have the factory +2 and it works fine but a full single tubes ideal.