choke tube +1
Yup your probably looking at a choke tube, installed in the bbl.
You don't want to defend our home with a "sawed off" shotgun anyhow. The lawyer types will have a field day w/ that (not right, but the way of things I regret to say).
Lots of factory short tubes as previously noted. Mossberg makes an 18" bead sighted tube for the Rem 870 which can usually be found for under $100 bucks new.
Mag extension? I had trouble with a tac-star and had to have the end cap re welded. SD/HD guns stay loaded and there's a lot of pressure on the mag spring, end cap and shell stops. My issue Rem ext has given no problem. I'm a big believer in the spring guide, which robs you of 1 rd, but coils the spring neatly for reliability, and since you cannot cram the mag to the gills, extends spring life as well, I think.