The front sight base on an AR is normally pinned to the barrel. The standard method is to drill the FSB and barrel together and then drive a solid tapered pin into place so the FSB doesn't move. For very high accuracy applications (Krieger barreled uppers intended for use in Service Rifle, or for knocking the eyes out of gnats at 300 yards with a varmint rifle) the base and barrel is sometimes drilled and a hollow roll pin or pins are used instead of the tapered pin. According to Clint McKee at Fulton Armory this is because the solid tapered pin actually displaces metal and can at least theoretically affect the accuracy. Not a problem with a chrome lined barrel but with a Krieger barrel I can see the concern. I have used a NM upper with a hollow roll pin and it never came loose, so that seems to work ok, at least for match use.
The problem is that sometimes people get the great idea that they can use a set screw and skip the pin entirely. Don't do it.
I know a guy with a DPMS LR308 AP4. It came with a quad rail from the factory and it had a standard FSB. He wanted a folding front sight so he let the shop he bought it from send it to their "gunsmith". The guy installed a railed block with two set screws and no pin.
At first everything seemed fine. He put 50 or maybe 100 rounds through it without trouble. Then it started to short stroke once in a while. Then it started not ejecting. He asked me about it and we took it apart and looked it over. The gas rings were present and it good shape. Heck, the bolt was actually tight in the carrier. The gas key was tight and the screws were properly staked. We ran a soft aluminum wire into the gas key and up the gas tube. Nothing was stuck in there. I did notice a slight gap between the back of the gas block and the front of the barrel shoulder but I've seen that before due to tolerances in gas blocks and barrels. The gap really wasn't that much so I just filed that thought away.
We oiled it and decided to try again. The first shot barely moved the carrier at all and on the second shot the carrier didn't move. I looked at the gas block again and noticed that the gap between it and the barrel shoulder was noticeably bigger than it was two shots ago.
The block wasn't pinned. Issue found.
Having the gas block drilled and pinned might cost $20 or $30 bucks but it is well worth it. If you are having one swapped out or you are custom ordering an upper be sure to have this done. If you are on a tight budget then wait another week or find somewhere else to shave $30 off the upper. You will be glad you did.
BTW this gas block was one that slips over the barrel from the front and has two set screws in the bottom to "secure" it. It must be drilled and pinned. Some gas blocks are of a split design that slides over the barrel and two or more screws clamp the block to the barrel. Some are two pieces that clamp around the barrel like a scope ring. The clamping types are better than the set screw type and if everything is degreased and roughed up they will probably work ok, but I would still prefer a pinned one. Then you don't have to worry about it coming loose at the wrong time.
Ironman's Pooper Potts
19 hours ago