I have a DD 16" LW middy and LaRue low-profile gas block. There is no relief cut (detent) in the barrel that corresponds to the set screw(s) in the gas block. Is it necessary to relieve a flat area on the barrel before installing the gas block? This is my first upper build so I apologize in advance to all the experts who have seen this question asked a million times before.
– ale
Not sure what you are asking. I assume you mean the barrel isn’t dimpled for the set screws.
OP,
I assume you are wanting to create a place to index the set screws on the LT GB. For the record, it is my opinion that a “flat spot” is notaufficient to meep the GB from moving. If that’s the case, Ranier Arms and Brownells both sell a gas block dimpling tool/jig. It indexes off the gas port so your block will be upright (provided the gas port is correctly located). Mine was from Ranier and it has 2 drill ports on the bottom to match up with several LP GBs on the market including the LaRue as that is the GB that I chose as well. It also has flats on the side for clamping in the mill vise.
Good luck! Let us know how it goes.
Sparky
Depending on your mechanical inclination and knowledge, competence and confidence with the AR platform, post #35 on the link below is a quick, inexpensive way to reference mark and dimple your barrel for a gasblock.
http://m4carbine.net/showthread.php?t=118884&page=2
If not, the BRDE jig suggested by Sparky5019 is your second option.
Third option is to send it out to Rainier or ADCO to get the work done, in which case, you might as well do away with the dimples and get it taper-pinned.
Yup, the dimple in the barrel is what I am talking about. I have a Bridgeport with DROs so indexing the flat shouldn’t be any problem. I just need to know if milling a flat is necessary or if the two set screws (by themselves) will hold everything together.
Also, thanks for the quick responses! I’ve been suffering from the flu this past week and having a hard time expressing my thoughts, hence my poor description of the problem.
– ale
The Colonel makes a good point about pinning. If you have the tools and the tech skill, I’d go ahead and dimple. As I said before I do not think a flat is a wise choice. The dimples pictured in the link the Colonel posted give you an idea of depth that is necessary to get the set screws to be secure enough to keep the GB from moving; a flat is simply not enough.
Sparky
GTG. Not sure if you need this but instructions from BRDE recommend a 11/64" (4.3mm) Drill Bit and drilling to the depth of its tip MAX.
I’d put a single dimple opposite the gas port.
Its main purpose is to align the gas block properly.
You can use rocksett on the set screws or stake them.
Pinning the block is a good option too.
Thanks for the advise, gang. I can easily go with a drill bit (dimple) versus end mill (flat). I’ll examine the set screws from my LaRue GB and try to copy the profile. Is it really necessary to stake these after applying red loctite?
– ale
If you have the ability/machine to do it, just pin the gas block in place and never worry about it.
The jig you guy’s are refering to is for a .750 gas block, not for the .625 block i believe he will be using on the lw dd barrel
I had one made at work to do the .625 lp gb
Good catch, MtC ! ! You guys had me a little worried with the GB dim…
Anyway, I’ve decided to put any milling on hold until tomorrow. I’m still a little groggy from the flu and don’t want to scrap any parts. The set screws appear to be .450" on center so it should be a straight forward operation. As we all know, setup is 90% of the work!
Somewhat related to this topic (uppers), I need to purchase some moly grease for the barrel nut. Everyone on M4C recommends Aeroshell 33MS but is there any downside to using one of the automotive brands? I came across Coastal Premium Moly EP Grease at AutoZone and was wondering if this would be an acceptable alternative? I can hold out for the good stuff if necessary.
Thanks again for all the quick responses to my questions!
– ale
On the last rifle I put together instead of dimpling the barrel, I staked the set screws on the gas block so they can’t back out. Still holding strong after 500 rounds.
Yep. I’ve got BRDE’s dimple jig and it works great. Anyone make one for a .625 barrel or a sleeve for .75 jig?
-Jax
While doing a search for “pinned low profile gas block” I ran across this image:
Has anyone else tried this? It seems to me that two set screws (with dimples) on the bottom plus a horizontal cross pin along the top would make for a bullet-proof setup, but what do I know. lol
– ale
Yes,
This is what everyone has been referring to by pinning it.
The only difference is the pin is usually on the bottom rather than the top.
Like this
Thanks, Clint. I’m new to the upper building world so I’m sorry if my questions make it appear like I’m in grade school. lol…
OTOH, I may have just found a way to re-purpose the roll pin from my factory trigger guard I recently replaced 
It’s not a question of backing out. The purpose of the dimple is so your set screw has something to fit and press into and not just onto, reducing, if not eliminating, the chances of it rotating, granted the set screw doesn’t back out. In which case, one could could then opt stake the set screws.
Jax: Im not sure about the availability of a .625 jig but after seeing CoryCop’s method, I’m now finding my dimple jig useless. I honestly never thought of that before.
ETA: Ale, I wouldn’t use a roll pin. You need either a straight pin (not sure what spec), or a 2/0 taper pin, for which you need a #31 bit and a 2/0 reamer.
Whomever did this need his ass kicked.
The pin should be on the bottom so it pulls the gas block down over the port… not on the top so it pulls the gas block off of the port.
On an SBR build, a couple of years ago, I used a BCM barrel (comes with setscrew dimple), then ordered a DD Lite 9 rail, a VLTOR low profile gas block, and gunsmithing services from Ranier. I sent them the barrel, they secured the set screw and pinned the gas block. They even thought to slip the barrel nut on before they pinned it. It was pretty cheap, as I recall…$25.
http://www.rainierarms.com/?page=shop/detail&product_id=1655