Do I need one? Picked up a Police trade in 870 with a mag extension but it is missing the mag tube clamp. Looking on Midway USA they are $40. At that point I might be better off just buying the WC extension.

A lot of people go without the clamp. I prefer them but you may never have a problem.

I bought one of my Department used Sage conversion Remington Sportsman 12 shotguns with extended mag tube.   I bought a Tac Star weapon light many years ago and it came with a couple of barrel clamps.   I used one of these along with a sling swivel stud and works fine.  I also used a polymer barrel mag tube clamp which has held up very well but inexpensive.   There are several listed on Amazon.

https://www.amazon.com/Shotgun-Barrel-Extension-Flashlight-Holder/dp/B003Y7U0SI

If the extension is a Remington factory, a clamp is very necessary.
If you don't have a clamp, the magazine tube will unscrew from the tube collar because the clamp is what prevents the extension from unscrewing.

If you have a 2 or more extension a clamp really is necessary to support the extension and prevent it from getting knocked or broken off.

The extension is screwed to the guns magazine tube which is thin steel further weakened by the deep square threads.
If you bump or drop the gun and an unsupported extension is hit, it will break off of the guns magazine tube, ruining the guns mag tube threads.
The extension seems to never get damaged, just the guns tube.
With a Remington that means an expensive trip back to the factory to have the damaged tube removed, a new tube brazed on and the receiver refinished.

I have seen at least 3 shotguns with the extension either bent out of line or knocked entirely off the gun after a surprisingly light blow.
Look at it this way.... An extension makes a nice lever to break itself off or knock it out of line.
That renders the gun out of service.
A clamp is cheap insurance.  There really is no good reason not to use one.

Two birds, one stone

https://www.magpul.com/products/forward-sling-mount-rem-870-mossberg-500-590

Went with an ATI clamp for now. Probably going to replace the +2 with a +3 extension to flush up with the 20” barrel. This is just an old 870P Police trade in that I plan to keep in the truck.

Attached File

I have tried several clamp options - Magpul seems to be high value.
As noted, a wise decision on an 870.

I went ahead and ordered a +3 extension to go with the 20" barrel. So it should come with a clamp.