Is this barrel dimple too deep?

Printable View

06-30-18, 16:29
websniper

Is this barrel dimple too deep?

As the title states, I'm slightly concerned about the depth of this gas block dimple. It's my fault. The QPQ finish was a bitch to get through and it cut pretty quickly after the bit made it through that. I should have taken it slower.

Anyway, the barrel is .750 at the gas block journal and it's a .300 AAC Blackout, so I assume .308 diameter bore. The dimple is approximately .080 in depth at the center (generous estimate, it is certainly no deeper than that). By my rough calculation, that should be roughly .141 inches of material remaining. Is this in any way unsafe or problematic?

I'm a little OCD about this kind of thing. If it's a problem, I'd rather scrap it and make it right (replace the barrel) rather than take any chances. If it's merely a cosmetic issue at this point I'll make my peace with it.

There is a slight amount of "wall" to the would-be dimple. I touched it up with cold blue, it's easier to see the depth.

https://i.imgur.com/H0sAfie.jpg

https://i.imgur.com/YaZL2j3.jpg
06-30-18, 16:49
Eric D.

I think it’s ok. A cross pinned gas block would cut at least that much depth out of the barrel; probably even more if someone wasn’t paying attention. Bore pressure also drops off rapidly on .300 blk. Just don’t go overboard torquing your set screw.

Next time use a grinder to take of the qpq/melonite/nitride and then drill your dimple.
06-30-18, 17:30
gaijin

I would be unconcerned with it.
06-30-18, 18:18
websniper

Thanks for the feedback and suggestions. I will proceed as planned with this barrel. Removing the QPQ/melonite first sounds like a good idea too.
06-30-18, 19:18
elephantrider

It is perfectly fine. A hair deeper than I would go, but just fine. There is more than enough depth left there. Someone once posted a 3D stress model of some 'worst case' situation with regards to dimple depth and screw torque. The conclusion was that even on a 0.625" diameter barrel, one would have to drill ridiculously deep and overtorque the screw to affect the internal bore. You are nowhere near that.
As for getting through the nitride; just be sure to use a god sharp coated drill bit. It can be a little slow at first, but goes easy once you get to about 60-70% of the drill diameter cut. Go slow, check frequently, and use a little oil. If you can see nearly, or 100% of the jig/drill diameter cut, then you've gone deep enough.
07-01-18, 11:31
khc3

Or get yourself a carbide spot drill, they are very handy to have.
07-01-18, 11:51
drtywk

Geissele recommends McMaster-Carr part number 28765A75 for drilling the dimples for their SGB. I have one and it makes quick work of QPQ barrels, or any barrel for that matter. Plus they are relatively inexpensive, $2.51 plus shipping/tax depending on where you live.
07-01-18, 15:11
websniper

Quote:

Originally Posted by drtywk

Geissele recommends McMaster-Carr part number 28765A75 for drilling the dimples for their SGB. I have one and it makes quick work of QPQ barrels, or any barrel for that matter. Plus they are relatively inexpensive, $2.51 plus shipping/tax depending on where you live.

Thanks for that info, I'll look into grabbing one of those for the next time I need to dimple.