Tutorial: Barrel Dimpling for low profile Gas Blocks

[mtdawg169]

When your local LE has to use a junk upper on duty, because it's that or nothing at all, you fix it and hope it gets put to good use one day.

Great example of what can go wrong with something as simple as a gas block install. Nitride barrel, cheapo low profile gas block with cup tipped screws. The barrel is so hard that the screws can't bite into it. So it moved forward and the gun became a one-shot wonder. Removed gas block, dimpled barrel, replace screws with aggressive knurl tipped screws, reinstall with red loctite. It's not going anywhere now!

This topic came up recently elsewhere and I offered to dimple a gentleman's barrel for a low profile gas block install. So, here's a little tutorial.

OK, here we go...

Using a Geissele gas block. The set screw spacing on the most common gas blocks is usually either 0.40 or 0.45". The BRDE jig is set up for 0.45 spacing. However, the SLR Rifleworks jigs allow for both by simply flipping the jig over and swapping the screws.

One thing to note here is that the port on the Geissele block is fairly small, so there is little room for error in terms of alignment. BCM & Vltor blocks have a significantly larger port which gives you more leeway. As a result, using a jig in this case is especially useful.

Distance from edge of block to leading edge of gas block port

Compared to the gas port on the barrel

Here, you can see the port aligns directly with the rear set screw hole.

The jig. 0.40 side facing up. The jig has two screws. One has a pointed tip, that indexes and self-centers on the gas port. The other, is a large cap head screw with a guide hole drilled right down the center.

Alignment screw with jig installed on the barrel

Guide screw

Installed in the vice

I don't have three hands, so no action shots. Here's the finished product.
Totally ruined...
***NOTE*** two dimples are not required for a secure fit. I usually do only one because gas block manufacturers use different length gas blocks and screw spacing can vary between makers. The forward (muzzle end) screw can vary in location. The rear screw will always be aligned directly below the gas port.

 

[mtdawg169]

Perfectly aligned

And the finishing touch is using the correct set screws with an aggressive knurling on the tip

I found this turd on the floor after dropping the set screw shown above. This is an example of what NOT to use. I'm pretty sure I threw it on the floor after swapping it for a knurled tip screw a couple of months ago. It came off of a PD gun that didn't cycle. I'll give you one guess why it didn't run.
Hint: look at Post #1

Duce's barrel in all of its engraved (and dimpled) glory!

Also interesting, here you can see that the Geissele block is compatible with a barrel pin. You can see that it would engage about half the width of the pin, which I prefer over a deeper set pin that drills completely through the barrel. Because: reasons.

Annnd, we're done!

 

[LoneOak]

I have that same jig and have used it a couple times. I need the opportunity to use it some more.

I read somewhere that when installing the gas block that you were supposed to leave that little space where the handguard cap goes on M4 style 2 piece handguards. Most of the time we encounter this is when changing from a cheap plastic handguard to a nice free float style handguard and installing a lo-profile gas block. I know the cap is eliminated in this situation but the original was mounted with this space so shouldn't a new one go in the same spot.

I sure wish I could remember where I read this, it might have been in the instructions for the SLG jig or I saw it a video somewhere. It just looks like in your pic that when you installed the jig and did the drilling you had left the space but when you installed the gas block it was gone. Maybe its something just as simple as your gas block is a little longer and the space wasn't required.

 

[mtdawg169]

I have that same jig and have used it a couple times. I need the opportunity to use it some more.

I read somewhere that when installing the gas block that you were supposed to leave that little space where the handguard cap goes on M4 style 2 piece handguards. Most of the time we encounter this is when changing from a cheap plastic handguard to a nice free float style handguard and installing a lo-profile gas block. I know the cap is eliminated in this situation but the original was mounted with this space so shouldn't a new one go in the same spot.

I sure wish I could remember where I read this, it might have been in the instructions for the SLG jig or I saw it a video somewhere. It just looks like in your pic that when you installed the jig and did the drilling you had left the space but when you installed the gas block it was gone. Maybe its something just as simple as your gas block is a little longer and the space wasn't required.

Leaving the gap used to be the absolute rule. These days, some manufacturers make their block for a flush fit and others are spaced for the gap. Using the jig, keeps it simple. The dimple is aligned opposite the gas port and ensures alignment every time.

 

[LoneOak]

So I guess the rule of thumb would be to use the jig to drill the dimples, install the gas block, tighten the set screws into the dimples and don't worry about whether or not you have the space. As long as your holes line up in the barrel and block your good to go. Some newer gas blocks may be long enough that there is no space but some may leave a space.
Would you agree with this, I need to know for sure! The guy I built this one for is already talking about another one.

 

[mtdawg169]

So I guess the rule of thumb would be to use the jig to drill the dimples, install the gas block, tighten the set screws into the dimples and don't worry about whether or not you have the space. As long as your holes line up in the barrel and block your good to go. Some newer gas blocks may be long enough that there is no space but some may leave a space.
Would you agree with this, I need to know for sure! The guy I built this one for is already talking about another one.

Most if not all gas blocks are set up so that the rear screw is directly opposite of the gas port. As long as this is true, you are orienting the jig correctly AND you make sure that you are drilling centered, it will self align. HOWEVER, all gas block screw spacing is not the same. Always drill the rear dimple first. The second dimple is optional. Just be sure that the spacing matches the block you are using.

 

[LoneOak]

Thanks for the info mtdawg!

 

[joe8845]

I know this thread is old, but anyone willing to dimple my barrel??? This is my first AR pistol build!

 

[LoneOak]

Where are you joe? Bring it over to the LoneOak Terrace and I'll be glad to do it.

 

[joe8845]

Where are you joe? Bring it over to the LoneOak Terrace and I'll be glad to do it.

Lone Oak Terrace Lithonia? I am located in Kennesaw. PM me an address and number and I will gladly come by!