Sometimes the air bottle is very close to the inside of the stock and you may need to use a larger diameter drill bit (I used a sharp spade bit) to accommodate the diameter of the large part of the Tee Nut. I also had to grind the down a tad so that they didn't protrude too far. This meant that I had to grind down the screws that came with the pic rail so that they didn't touch the air bottle and mar/scar it up. The first nut, Knurled in the table above, would not have worked as the flange is too thick. The Tee Nut flange is about half the thikcness, so that made my choice easier. It took some time but I have just enough clearance to slide a business card between the stock and the air bottle....it is that tight.
Something so stinkin simple becomes so stinkin tough to do sometimes.
mike