Like the others have stated any adjustable mil-spec Carbine sized tube will work. Just in case? Be aware if you are planning on using a stock and not a brace? Then you would be turning this into a SBR and you will need a tax stamp etc. Same goes for a verticl fore grip this also needs a tax stamp. Stay safe!

Dave, this just has the non slotted Pistol Tube installed. You will need to replace it with an adjustable tube : - https://ar15discounts.com/products/trinity-force-mil-spec-carbine-buffer-receiver-extension-kit/

Dave, if you are talking about the Magpul arm brace you would need an adjustable buffer tube. The brace won't fit on the tube the pistol has installed. Any milspec carbine buffer tube would work. Hope this helps