12 Ga flush barrel / +2 mag extension ?

[Laddk84]

I recently purchased a 870 synthetic with the 28'' vent rib barrel for trap shooting.I also wanted to use the gun for home defense, This is where i need some help. I want to add a Remington +2 mag extension which specs say is 6.25''. I am unclear on which barrel i need to buy to make the mag tube an barrel flush? I've tried to research this but have heard both 18'' and 18.5'' are flush???I don't know if aftermarket barrel measurements are misleading or if there measuring the whole barrel or just from where it starts from the chamber??? I was leaning towards a Carlson's 18.5'' barrel with the Rem choke system, i like the idea of being able to change chokes, but if i have to go with a 18'' barrel i don't think i can get it with the Rem choke system? Can anyone with experience with this please set me straight on which barrel will be completely flush or possibly some pics an measurements?

Thanks

[Synchronizor]

The Remington factory +2 extension tube is 6 7/16" long, and since it's a two-piece extension, this length is added to the end of your gun's magazine tube. It will line up flush with an 18.5" barrel.