If I have a barrel that I want to put a low profile Vltor GB or similar on, how do I locate the spot for drilling a dimple?
Thanks!
My guess (and only my guess as my only dealing w/ a lo pro GB is shaving down a FSB) would be to get the GB and then mark w/ a punch where the hole is located for the set screw to go into the barrel. Then remove the GB and drill your little dimple. As I said that would be my guess but I’m sure it’s not correct. 
It should be 180 degrees from the gas port.
The gas port is usually 0.295" from the step at the rear of the gas block seat, but measure your specific barrel and just match the distance to the gas port distance.
As for getting it perfectly bottom dead center on the barrel, a quick and dirty way is to clamp the whole upper in the mill vise with the flattop flat relative to the mill table.
That way you know that you have the barrel perfectly upside down to drill the detent.
Then the gas block ends up perfectly upright.
I charge $10 for a detent job if anyone needs it done right.
Ok, let me elaborate-
How do you do this with a vice and drill or drill press ???
I know the dimples go 180d opposite the port but thanks for the distance from the step.
I was hoping for an intuitive way of centering the barrel in a vice clamp in my DP and also getting it 180d off the port…
Do low profile GBs usually cover both FSB holes?
Just eyeball the gas block straight on the barrel.
Tighten the rear setscrew really tight.
It should be a cup-point setscrew so it will leave a ring impression on the barrel.
Remove the gas block.
Center punch the center of the ring.
When you drill the detent, just drill into the center punch mark.
Without a mill, that’s the best idea I can give you…
And no more talk of dimpling.
You are talking about a detent.
After finishing this today, I am sick of dimpling…
The way I’ve been doing it lately it to make a sacrificial gas block of the same brand and model of the one you plan on using. My favorite lately is VLTOR. I have one that I cut the top off of it so that I can see the gas port in the block. I install the sacrificial block on the barrel and tighten down the rear screw very tightly after confirming the gas port in the gas block is centered side-to-side and front-to-back on the barrels gas port. Then I drill a dimple through the front set screws hole. Remove the sacrificial gas block and install the real gas block. Usually I only dimple one (the front) and then use red Loc-Tite or Rocksett on the set screws. Dimpling both holes causes (since I don’t have a drill press) the two set screws to fight each other causing the block to sometimes move.
Aren’t detents more like cylindrical depressions, straight walled? Or actually, when thinking of a ball bearing detent, something that is made to catch?
I have been calling it dimpling because I plan on only having a dimple in the end!
Will I be drilling deeper than that?
My WOA from ADCO only had a dimple really.
I had a lot of problems lining that little bugger up after taking it off once- set screw FSB and all…
I’m worried that I’ll remove the BCM standard FSB and ruin a perfectly good and 100% reliable upper if I don’t get it perfect.
I may just be cutting off the FSB.
Will an Vltor cover stock FSB pin holes?
GotM4- your idea is very good and if I were to do this with several barrels, I may think about doing it. In all actuality, what I’D personally do is get an aluminum block, 1"x1" or so and 1" long bored out to the barrel OD and drill a hole through it the correct distance from one end the size of the gas port. I’d slip it over the barrel and install a temporary pin to hold the block in place. Then I’d drill a tiny pilot hole through the hole on the opposite side, locating a perfect 180d offset hole.
But I don’t have a block of aluminum or a 3/4" drill bit 
A detent is simple a small depression designed for something to catch in it.
It could also be the ‘something’ such as the case of the takedown and pivot pin detents.
In the case of a gas block, you only need to drill a 0.150" diameter hole that’s JUST deep enough to have the whole front end of the drill bit cutting.
You don’t need to drill deep enough to end up with straight sides to the hole.
If you want it perfect, pay someone else that you trust more than yourself to do it and quit worrying about it.
Well I installed a Dawson rail on my SA Loaded and built an underfolder so I ought to just quit worrying.
But, the important question still remains-
What will cover those FSB pin holes?
LaRue, Samson and MI all make low profile gas blocks that cover the pin holes in the barrel.
Ok. So an Vltor won’t. I think I’m just going to cut the FSB and get a 10" DD to fully cover it.
FWIW I’m a big fan of cut down FSBs if you can make them work. I believe that re-using the taper pins is much better than any amount of dimpling/detenting/set-screwing.
I agree a pinned gas block is the absolute best. Sometimes it’s pretty hard to grind them down far enough to fit under certain rails (LaRue and DD Lite), in situations is when I use a VLTOR gas block.
I’ve repaired 2 S&W Tacticals where the factory low profile gas block wasn’t dimpled and moved.
That’s one of the chief reasons I use the standard DD rails.
I’m actually about to start grinding on a FSB shortly. It pains me a bit as it’s an “F” fitted to the barrel but I can’t get the 9.5 FSP to do what I want it to so I don’t have much choice.:mad:
Alright, I’ll be grinding the standard FSB.
I planned on using a Troy MRFM (? the midlength) but will probably use a DD 10.0 or 9.0"
I like the 9.0 because it stops almost dead even with the front of the FSB.
That looks a lot like work I’ve seen from KAC. What’s something like that cost Randall?
That’s what I did on my wifes and my 3gun uppers. I cut the top off of them and ground them down, cleaned them up with a belt sander then sandblasted them and finished them in satin Moly GunKote. They came out pretty nice looking. I wanted these as reliable as any ‘fighting’ rifle and neither has given us any trouble whatsoever.
My 1A carbine is set up that way with a DD9.0 over the top of it. ADCO did the work for me on that one. I’m going to attempt this one myself.
I don’t ever want to do one again.
At least not until I can get a CNC machine to do it for me.
There are 224 dimples there.
All were hand-cut on the bridgeport with a carbide ball endmill.
I did that one (my first) for $200, but I would have to charge at least $300 in the future.
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