I want to change the SB15 on my P516 to a real stock like the Magpul UBR Collapsible Stock due to wanting to shoulder the weapon and because of the "cool factor".

https://www.magpul.com/products/ubr-collapsible-stock

Here is where it gets weird. The UBR comes with the necessary Entry-Length Receiver Extension tube. My question is will it fit on the receiver of the Sig.
An image of the RE is here...
http://i355.photobucket.com/albums/r...bum/Sig-RE.jpg

I measured the tube where the Brace slides on up to the hex shaped stop at a rather oversized 1.220". I measured the tube ahead of the hex near the Sig logo at 1.120". And the threads at 1.175". Certainly not a standard buffer tube.
I did take out the spring and buffer and they measure standard mil-spec carbine size.
If I get some other stock, what buffer tube would I need?
And yes, I've SBR form 1 the pistol. I did this because I did not want to be at the range, shoulder the SB15, and have a conflict with some officer of the law! Whole other can of worms.