what stock and how are u mounting to sbs your tac 13s? I have seen pics but not much info out there.

Think any V3 stock will fit. CDNN had some closeout wood furniture but I'm not sure if the forend is the same.

I used a Magpul 870 stock with the adapter plate that comes with the TAC-13.  It puts the dot in exactly the right place for me.  If you go that route, you'll need to replace the bolt that comes with the stock with a 1/4-28 x 2-1/2" socket head bolt.  I've posted this picture here before, but what the hell:
Attached File

Have you ever seen that adapter separately? I've heard about it but never seen one.

Nope, but never really looked either.  I just checked and the manual that came with my TAC13 is the standard Remington V3 manual.  It calls out a "stock shim" in the IPL but doesn't give a part number, and I'm not sure it's the same part that comes installed on the TAC13.

I think I still have mine somewhere.  I SBSd my tac13 with standard V3 furniture so I didn't use the factory plate.  I can take a look for it tomorrow.

ETA: found it faster than I expected.

ALASKANFIRE. The adapter you are inquiring about only came with the Tac-13. I searched the planet over as well as a couple other planets coming up with ZERO.
I inquired with Remington prior to their demise and they did not have a listing or PN for it. Muzap's pic is what that adapter's most obvious use will be. Fits the Shockwave Robinshead/Rapter grip as well.
I think it was designed for that purpose only but Certain 870 stocks will work well on it.
I hope to solve the issue very soon. I hope to have the prototype completed and in the final stages of a small run production of around 100 pcs. If those go out quickly, I will do more.
The biggest issue with the part is that it has so many angles and steps to create. Not a good good candidate for CNC work. Cast maybe being the best as I think the original was cast.
Muzap's pic really show how well the Magpul SGA stock really works with the V3. I have already completed an 11 inch V3 using my one and only Tac-13 adapter with the SGA stock for the 870.
It's since become a folder with a different adapter, different Magpul buttstock, and it looks great and functions just as well. I'll post again when I get some adapters completed.

Update: I have had the original adapter scanned for printing and had some printed using Nylon12.
I am running a durability test on one of these and am approaching 1000 rounds using the Magpul SGA stock for the 870 on a V3 Field Sport shooting trap.
It is looking very good and I will post some pics once I reach the desired durability test limit I set.

I have run 900 rounds plus with the adapter I had printed. It has held up well. I also added a 10-32 set screw to the bottom tab to help keep the alignment vertically with the Magpul SGA 870 stock.
The adapters were made from Nylon 12. Nylon12 is a durable material and it has performed extremely well. The fit and texture is excellent. Tested shooting trap and field loads. It is paintable as well.
Here are some pics.

https://i.imgur.com/ki3mYWc.jpg
https://i.imgur.com/fFOqAQ9.jpg
https://i.imgur.com/RMVp3BW.jpg
https://i.imgur.com/nKIyUVg.jpg
https://i.imgur.com/zNrJO7B.jpg
https://i.imgur.com/AiDKhfQ.jpg

Muzap. I had some of the Remington V3 Tac 13 adapters printed using Nylon12. I made a nice modification to the adapter that holds the SGA stock higher up on the adapter.
I drilled and tapped the adapter for a 10-32 set screw in the bottom tap of the adapter. Located to push the SGA stock up to make the top surfaces match so there isn't such a big step at the interface.
This pic is in one of my adapters installed with the SGA stock. I have done it to the original steel adapter as well. Greatly helps the interface. Locate as I have if you decide to do this.
Just a tip. I drilled the tap drill hole at .156 instead of the required .159. Keeps the set screw snug.

https://i.imgur.com/AiDKhfQ.jpg