Have run a Truglo Gobble Dot on my 26" 870 3.5" Supermag for at least 15 seasons and have been 1k% happy with it. Fit perfectly and very securely on my vent rib.
Its all steel and nearly bomb-proof durable and has never worked loose once set.
I have no doubt what so ever the Truglo GD sight has allowed me to shoot more accurately, allowed me to adjust my patterns POI to match my POA and allowed me to extend my maximum ethical range.
My friends who based on my success with my Supermag turkey hunting also bought Supermags and Truglo Gobble Dot sights and all have been absolutely happy with their performance.
My ONLY complaint is a very recent one of the last 3 seasons and is a issue based on my shortcomings more than of the sight system itself.
Now that I am in my late 50's my eyesight is having issues seeing the fiber optic fiber of the front sight. I do wish it's OD was about 2-2.5x larger.
Other than that the sight is awesome.
If your barrel has a vent rib on top IMO/IME you can not beat the bolt on Truglo Gobble dot sight.