The laser mounted in the mag tube worked well except you loose a round so a 4+1 870 becomes a 3+1.
It will not work if you wish to attach a sling using the threaded whole in the mag cap.
Controls for the laser are next to the small hole in cap so you can turn the laser on an cover the hole with your finger until you need the laser for a accuracy.
Weak point in all this is the on/off switch and difficulty mounting a sling if you so desire to.
BTW, this has some potential to work well on a PGO as you could shot from the hip etc with accuracy.