I know why they send you a roll pin - AIM Sports Inc .750 Low Profile Gas Block/Mil-Spec

Most gas blocks don't come with a roll pin, I know why this one does. They give you one so that when you ruin it, you still have the one that comes with your gas tube to use on the replacement gas block. I got the roll pin started but it would not go all the way through. I finally figured out why, the holes don't line up and the roll pin hits the other side and no amount of trying to rotate the gas tube to get the pin to line up so you can get it through. Another thing, the hole in the gas tube for the gas to travel does not line up with the hole in the block properly. Looking through one of the set screw holes you could see that the front part of the hole in the tube was being covered by the front part of the gas block. It would have probably worked but it was definitely out of spec. I bought this because I had two barrels I was trying to build uppers with and I needed another gas block. When I used the roll pin that came with my gas tube (pistol length) it was installing the same way and I was careful not to ruin it. I backed it out and retrieved my other gas block. Low and behold, same gas tube and roll pin went in the first time with no issues and the gas hole in the tube lined up perfectly on the second gas block. My experience with this gas block was a waste of time and money. Mine was definitely not Mil-Spec. I may have got a bad one but fool me once, shame on you, fool me twice, shame on me. I am just glad I am only out $7. Lesson learned, I don't recommend buying one of these.