Barrel Dimpling - the Why and the How.

Howard Law

DarthVader

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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.

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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.
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Distance from edge of block to leading edge of gas block port
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Compared to the gas port on the barrel
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Here, you can see the port aligns directly with the rear set screw hole.
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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.
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Alignment screw with jig installed on the barrel
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Guide screw
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Installed in the vice
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I don't have three hands, so no action shots. Here's the finished product.
Totally ruined...
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Perfectly aligned
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And the finishing touch is using the correct set screws with an aggressive knurling on the tip
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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
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Duce's barrel in all of its engraved (and dimpled) glory!
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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.
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Annnd, we're done!
 

NWS

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I had the gas block walk off the front of the gun while shooting full auto a couple of months back. You know, my suppressor doesn't do shit when the gas block moves forward.
 
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cbh13

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excellent post! have to dimple one this weekend myself but a build im finishing up
 

DarthVader

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I had the gas block walk off the front of the gun while shooting full auto a couple of months back. You know, my suppressor doesn't do shit when the gas block moves forward.
Yeah, that does tend to happen! Something about that whole gas impingement system thing? I dont know...
 

DarthVader

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Mar 25, 2015
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I had the gas block walk off the front of the gun while shooting full auto a couple of months back. You know, my suppressor doesn't do shit when the gas block moves forward.
What's funny, is that according to some, that never happens, dimpling the barrel is stupid and apparently ruins the barrel. I mean, just imagine... What if that full auto gun went down while the zombie hoard was closing in on you?

The very first picture in this thread is the reason I recommend it. That gun currently rides in a SWAT officer's cruiser, sitting on a Colt M16 lower. He can't afford "what ifs". And I have the same opinion of my personal guns. Anything I can do to improve reliability and durability, is a good thing.
 

NWS

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What's funny, is that according to some, that never happens, dimpling the barrel is stupid and apparently ruins the barrel. I mean, just imagine... What if that full auto gun went down while the zombie hoard was closing in on you?

The very first picture in this thread is the reason I recommend it. That gun currently rides in a SWAT officer's cruiser, sitting on a Colt M16 lower. He can't afford "what ifs". And I have the same opinion of my personal guns. Anything I can do to improve reliability and durability, is a good thing.


Honestly I have never had it happen on a semi auto gun. The full auto lower will walk a gas block off in just a few mags though if it isn't installed properly. I'm sure it has more to do with the amount of heat full auto creates. I prefer my gas block pinned, but will settle for dimpled until I can get it pinned.
 
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DarthVader

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I've done that with a vltor (or maybe DD) clamp-on that was torqued up and loctited. It didn't move.
OK. But consider this, after heavy use, lots of high heat cycles, etc the set screw / dimple will still be more resistant to a lateral blow or really any movement over time. Because the set screw is seated below the barrel surface, it creates a mechanical attachment + tension attachment. The set screws themselves lock into the dimple walls, preventing any shifting. Clamp on or a set screw + no dimple is only retained by tension. Sure, they work, but they are not as secure.

A taper pin + dimpled set screw is the most secure method. But it's also the most difficult to do correctly.