AR gas block dimples?

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First time building an AR upper. Barrel has one dimple for the rearward gas block set screw. Is that typical and all that's needed? Or am I supposed to drill a second dimple for the forward set screw? Not sure what the standard is here...

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First time building an AR upper. Barrel has one dimple for the rearward gas block set screw. Is that typical and all that's needed? Or am I supposed to drill a second dimple for the forward set screw? Not sure what the standard is here...

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I could be wrong, but on mine it seemed more like a reference. For lining up the gas tube, and giving a warm fuzzy feeling you have it sitting correctly.
 
Correct me if I'm wrong since I'm also new to upper assembly.

Standard is one dimpled, two would be better but not necessary. Factory only dimpled one since gas block set screws distance differ from each manufacturer.

On my lightweight AR build, Faxon 14.5" had no dimpled. I got SLR dimpling jig and dimpled only one. Faxon gas block has three set screws, I torx to spec with red threadlock.
 
If the barrel doesn't have any dimples, I position the gas tube correctly and tighten it down so that the set screws leave marks on the barrel. I then remove the gas block and use a file create short channels that correspond to where set screws will come to rest. It achieves the same effect as dimples, but you don't need a jig and it's super easy.
 
yes, add a second dimple for the second set screw. Do it after you mock it up and line the gas tube up.

The reason you only have one is because the barrel maker can't know where the second one would be.
No standard exists, since the only actual standard is the A2 front sight/gas block assembly. It has only one hole, between the sling swivels, used as a clearance hole for drilling the gas port passage in the sight. Service rifle NM sights use that hole to retain the sights instead of the tapered pins and slots.

So a barrel maker will always know where the rearward dimple can be, opposite the gas port in the barrel.
 
You have wandered into 9mm vs .45ACP territory... If you are going to dimple both screw positions, it is best to have a jig. You can use the second gas block set screw hole but you risk damaging the threads or making an off-center dimple.

There is no standard for gas block set screw spacing. Therefore, if you dimple both screw positions, you could only replace the gas block with another gas block which has the same screw spacing. The dimple that came on your barrel is directly opposite the gas block port and will both align the gas block correctly and prevent rotation of the gas block. Many internet experts opine that a second dimple is superfluous, limits future upgrades and should never be drilled. Other OCD sufferers insist that only two dimples will prevent the gas block from moving under heavy use in the inevitable zombie apocalypse. I have done it both ways. depending on where I fall on the obsessive-rational spectrum in the moment.

If you are going to do more upper builds, you will probably want to invest in dimpling jigs since most of the barrels I have seen are not pre-dimpled. You just have to make sure to get a jig which is for the particular gas journal size of the barrel. .750 (government profile) and .625 (pencil/light weight) seem to be the most common. I have bought the KAK jigs in those sizes and they work fine.
 
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I decided to assemble it as is with just the one dimple. I'll just keep an eye on it.

Getting a dimpling jig is probably a good idea.
 
Ive had a few where it was necessary to dimple the 2nd due to clearance issue with some super slimline handguards. 1 handguard I just milled the M-lock opening to clear it.
Anyway dipple jigs are cheap and easy to use. Plus should you wonder what the difference between dimpled an undimpled G/B when whacked with a 2.lb hammer or 3foot pipe wrench- you can do a scientific test and find out. Spoiler- dimples make a BIG DIFFERENCE.
 
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