I found one in a gun shop about a year ago. I paid 25.00 bucks for mine and didn't realize what a deal I had made. This was the rail made by Kel-Tec. Comes with instructions. The only down sides I are the gun will no longer fold in half. The rail is attached to the barrel by clamps. The clamps are made from aluminum and I found myself having to re-tighten the clamps every 50 to 60 rounds. The threads on the clamps eventually stripped out but a friend that worked in a machine shop was able to make me some identical ones out of steel. That cured that problem. I mounted my EOTECH on the top rail and I love it. Don't know if Kel-Tec still makes it but you can still see it on their website here: Kel-Tec CNC - SUB-2000 Accessories - SUB-2000 Aluminum Forend w/ Picatinny Rail I had heard that they were no longer available but that may have changed. Price is listed as 97.53 but you may be able to find one floating around at a better price.
Aside from the cheap aluminum barrel clamps it is a worthy accessory. Allowing the Sub2K to share all of my AR goodies. I recommend using blue Loctite for the barrel clamps.
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