I just finished a SBS and thought I would post the way I added a light, +1 extension, and sling to the 14" 870 Police. It is a local PD trade in that I got a good real on. I modified the forend with a Dremel to mount a Mapul cantilever rail section. It is secured with JB Weld and the single bolt is threaded in to the plastic. It is in the same location relative to the rear of the forend as the rail on my other 870 which has the MOE forend. I like the cantilever rail because of the way it tapers down to the forend. It is a good place to rest your thumb when the light isn't mounted. I didn't use the MOE forend on the SBS because of the way it overlaps the extension nut. I considered using the MOE and a Magpul Forward Sling Mount for QD. TeeRex has his set up like that and it works, but I wanted to keep it simple as well as low cost.

The extension is a Vang Comp +1 with a removable sling plate (secured with a snap ring). I left the plate on but reversed it so that it is on the right side, out of my way. I kept the plate to keep the 550 cord loop that secures the Proctor sling from sliding off the end. I used the proctor sling to keep it light, simple, and inexpensive. I will run the SBS in a class soon and if it works out, may grind the Vang plate down so it is round on both sides.

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Here is how I attached the 550 cord to the stock. The trade in had a Choate stock which I was able to cut down to 12.5" LOP.

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Here is the right side with the sling plate I will probably grind down.
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And here is the final product except for the 4 round GGG side saddle that will go on the left side of the receiver..
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The rail has two posts that require holes as well. The rear post is oval and I originally drilled it too low. I had to enlarge it at the top so the final reult is a hole bigger than necessary. The JB Weld and threaded bolt keep the whole thing secure. The white is a paper towel stuffed in the forend to keep the JB Weld from running out that the back. I drilled the bolt hole a little small and used the bolt to cut the threads.

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Very nice DIY work!

Great work - I'm very interested in your 550 sling attachments as I'm currently running Specters set up as two-points and they're heavy with all their hardware and material, most of which isn't needed and your set-up illustrates a good simple solution.

Your fore-end mod is also quite interesting.  I'd tried something similar on the front right leading edge of a standard polymer 870 police fore-end but it didn't work because the backing plate of the Streamlight attachment I was using would hit the barrel retaining ring not allowing the action to completely close.  I gave up at that point but your idea gives new direction.

I have a Vang Comp sling plate on our company shotgun and ran my thumb into it during a string of fire during a qualification earlier this year - it hurts a bit so watch your light once you get it mounted - I can see the same thing happening if one were to use a TLR-type light, an Inforce WML would likely not cause such an issue.  Recognizing your handle from TOS I realize you don't need my advice just pointing out something for which to watch.

Finally, I'd also like to know more about your stock shortening procedure - it appears that it worked very well.  I'm not familiar with the Choate stock but the standard polymer stocks on both 870s and 590A1s are hollow and seem like they'd be good candidates for this.  I'm not sure how a SpeedFeeds would do though as they've got brass buttplate screw retainers in them.

The Choate stock has a flat area at the rear on the bottom a little more than half an an inch long before the line angles up to the rear of the receiver. I was able to cut it off with a chop saw at the point where the angle changes, but I didn't get it quite square from left to right. I was able to square it up with a Dremel and sanding bit but an orbital or belt sander would have been better and faster.

The channels for the recoil pad screws are solid and extend well into the stock. The recoil pad has a plastic ring that sits in a relieved area inside the stock. I relieved the stock for the recoil pad with the Dremel. It was a simple matter to screw the recoil pad back on. The recoil pad is a little oversize now since the stock tapers, but not enough that I feel the need to grind it down.

After I run the gun in a class next month I may grind grind a little more length off with an orbital sander.