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Quick question:

When installing a low profile gas block on a typical AR-15, how necessary are the "dimples" in the bottom of the barrel? 

I've seen them on my BCM barrel, but after working with a couple Aero Precision barrels and a Larue one, none of the others had the dimples for the gas block set screws to go into.

Should I be drilling these somehow on those barrels that don't have them? What kind of jig or whatever should be used to ensure the gas block is straight up and down, and the holes are proper? 

Is red Loctite and tightening the fuck out of these "good enough" on barrels without the dimples?



Original Post

A gas block that is only screwed onto a non-dimpled barrel may or may not hold.  I've seen too many of them let loose, especially if something gets caught up in the charging handle track and pushes the gas tube forward. 

Thread locking agents like red & blue Loctite on the screws in theory will usually keep them from loose.  Heat soften Loctite, and gas blocks get hot, so there is the possibility of screws coming loose.  Even with thread locked screws on a non-dimpled barrel may not have enough resistance to hold if something gets into the charging handle track and pushes forward on the gas tube, I've seen them come loose.

Dimpling the barrel where the gas block screws mount onto the barrel helps, as you are basically creating a countersink hole for the screws to go into, so if something gets into the charging handle track and pushes forward on the gas tube, it will usually have enough resistance to hold things in place.  

My personal preference is to pin gas blocks.  In our Advanced AR15 / M16 Armorer Course we teach how to align/fixture/drill & pin gas blocks, how to gauge/fixture & dimpling barrels for gas block screws, and how to drill/ream/pin front sight bases, etc.   A pinned gas block will not come loose, unless the pin is removed, which is something that should be done on gas blocks if lives depend upon that rifle being 100% reliable (Imho).

Greg Sullivan "Sully"
SLR15 Rifles

(763) 712-0123


Based on the recommendations of people with way more experience than me, I've pinned all the low pro gas blocks I've installed. I used jigs to both dimple then pin them. I haven't put a significant amount of rounds through any of those guns yet, but initial function is positive. The dimple jig indexes off the barrel gas port with a pointed set screw so the gas block is perfectly centered. The dimples w/ set screws then ensure that the gas blocks stay aligned and don't move while I drill & pin them. The jigs are very easy to use with a little bit of mechanical inclination. They recommend using a mill or drill press, but I used an electric hand drill and it worked well.

Last edited by Community Member

I strongly agree on pinned gas blocks. I have seen/fixed issues with set screw and clamp gas blocks. I prefer clamping (not two-piece clamp down styles) over set screw, though, because I think they provide a ton of surface contact versus set screw block which tend to pull the block down from the top and thus away from the barrel on the bottom.

For set screws, I think dimpling is a must to hold in place (but not holding power, if that makes sense) especially if the block doesn't rest fully against the shoulder on the barrel, and for properly aligning/locating the block. If you want to dimple yours, and assuming it has two screws, there is a trick to aligning the block with the gas port:

Use a round toothpick or other similar round piece of wood or plastic that fits into the gas port loosely enough to slide, but not so loose as to wiggle excessively. Place inside the port (which should be cleaned and dry) and then flush cut it with the barrel using very sharp blade. Remove and score the pin at the halfway mark and then make sure the cut end is rounded/smoothed. Once set, check fit in gas port again. If satisfied, leave it in place.

Now place block onto barrel, slide into position, and then flip gun over. This should cause pin to drop into the gas block's gas port. The pin will be partially still in the barrel, as well. Now you can tighten the forward-most set screw in place. Once satisfied with alignment, flip gun back over, which should cause pin to drop out of gas block port and back fully into barrel. Use a cleaning rod inserted from chamber to break pin off at score mark and observe to ensure the first half comes out the front of the barrel. Pull rod back and repeat to remove second half.

Not you can use a small (1/8" or so) drill bit inserted through the gas block's rear set screw hole to mark the dimple location. The bit should fill up the screw hole but not so much to damage threads. Drill slow, use oil, and pre Drill in about 1/32" or so, but no more than 1/16". Remove the gas block. Using a drill press and ensuring the barrel is securely clamped in perfect vertical alignment with drill bit, drill out the pilot hole per gas block manufacturer's spec for the sec screw, likely a 3/32" diameter bit, drilled to about 1/16". (spitballing, so find out for sure based on your selected parts.)

That's my method, anyway, which I've used twice. There is this item as well, which might be preferable to screwing around with making a toothpick alignment pin: ar15-gas-block-aligner-5-56mm

I am inclined to stake the set screws when tight (can be a pain) and use red locktite, too. I've considered soldering them, I just don't trust gas blocks unless pinned, over abundance of caution and all, even under metal railed handguards. Heat, vibration, recoil, and gas pressure can make tight things loose. Threads can stretch, etc.

Last edited by Community Member

Another vote for pinning, but I'd at least use the set screws unless this is just a fun gun. 

I've bought 2 LaRue barrels this year through their sale, and both had 3 dimples for their gas block (an important consideration when most gas blocks only come with two set screw holes, and they have different spacing than LaRue's gas block).  One has a LaRue gas block, and the other has a SLR adjustable gas block that they made for me with LaRue's dimple pattern.

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