My current situation: I am accumulating parts to build my first SBR. I do not have the stamp yet so I am careful about what parts I have around. I bought a stripped PSA 10.5" barrel and a Daniel Defense MK12 LPGB. I sent the barrel with the gas block out to a reputable shop to be dimpled for the set screws of the gas block.
My question: the barrel will be delivered tomorrow and I would like to know is there any method used to verify that the gas block is correctly positioned over the gas port?
It is a reputable shop so I don’t have any serious doubts about the quality of their work. Yet at the same time, I don’t want to get my stamp, build the rifle and realize the gas block has been incorrectly positioned.
Also, like I said, I do not have my stamp at the moment so going out and shooting it wouldn’t work.
I believe if you take the set screw out, rotate the gb upside down, you should be able to see the gas port. In other words, the dimple should be right below the gas port. The opening into the gb is larger than the gas port so there is a little wiggle room.
Eric: ok, I’ll give it a try, but two issues: there are two set screws and I don’t know if either of them are in line with the gas port opening in the gas block, and I can’t verify if the gas block is properly positioned upside down either (right?)
Munch: I measured it when I sent it out, only because I wanted to know that it wasn’t too large or too small. It came out to around 0.078". I’ll measure it more precisely again when I get the barrel.
amd5007, the gas port in the gas block has to be aligned with one of the setscrew holes because they drill the gas port thru the screw hole. If you temporarily mark the gas block location on the barrel, remove the setscrews and rotate the gas block 180 degrees I’m sure you’ll find your gas port on the barrel. Sorry to be so long-winded. Hope this helps.
lowdowndad:cool:
Ok thank you, that is a useful bit of information.
But while this is a brand new barrel, the gas port on the barrel was already drilled at the factory (either PSA or FN). The shop is not drilling the gas port on the barrel, but rather dimpling the barrel to enable proper securing of the set screws.
I have come to understand so far that my risk of having an issue is low. I will try to play around and make sure everything matches up correctly though, (perhaps use a pipe cleaner and look down the barrel?). I was just curious if there was a standard method for determining the correct placement of the gas block over the port.
You can draw a line in line with the bore centered on the gas port using a sharp pencil. Locate the gas block center and align with the mark on the barrel. Also measure the distance of the seat shoulder to be precise. To save yourself the agony, and make it “bulletproof”, send it to ADCO to have the block pinned. There are also jigs made to correctly align, mark and drill the dimples. Normally, the block is off the shoulder approx the thickness of a hand guard end cap, that allows one to be installed if going that route and allow for the correct seating of the block or front sight base.
I won’t say who, but a dealer once sold me a barrel that he drilled himself and didn’t allow for the end cap, realized it and dimpled again giving me elongated holes. It was sent to ADCO anyhow for the pin which covered it up.
Ok thanks. I’ll pull out the calipers and pencil and measure.
By they way, I did send it to ADCO for the work. I trust them. I didn’t want to name-drop because I wanted to get a more technical answer rather than testimonies of they’re work.
I can say they’ve been right by me 6 times so far.
I’ve often wondered why barrels aren’t made with a spline and corresponding cuts in a block, you could truly get away with a clamp then. Or even decent flats on both. Done right, there would be no canted bases, etc.
Spoke with them to confirm, said they couldn’t release because it’s proprietary information. Did confirm that port size range was larger than the .059 - .064 I was looking for. If you could advise once you measure it’d be appreciated
.078 is a bit large for a 10.5". Noveske uses relatively large ports though and for whatever reason, they don’t release that info. Colt, DD and LMT are a good bit smaller with .068 being a decent average.
There was talk about Colt even using a gas port in the .090 range on their shorter barrels. I heard this as well, but either it’s not true or there’s more to it that I don’t know about.
I don’t believe any manufacturer uses .059 for a 10.5", and that’s even smaller than a 16". We’re in custom territory here in that range.
My LMT mk18 shoots almost as soft as my SR15 with the Vltor A5, whether it’s suppressed or not. With a carbine RE and an H buffer, it’s very comfortable as well, but you notice the difference with ammunition a little more and when suppressed.
I figured I was in the custom realm, per our discussion last week about a dedicated suppressed upper. But for $199, I figured I’d at least check to be sure. The hunt begins for a barrel to send to ADCO…
Plug the chamber end with your finger and blow into the muzzle end. If the air “escapes” it’s lined up. If you get resistance, the block is not clocked correctly.
Got the barrel today. I’ve tried pretty much every method listed in this thread and it has passed. Now I need to wait for the stamp and shoot it.
About the gas port: I don’t understand why they would tell you it’s confidential information considering you buy one, measure it and now it is public information. The diameter is around the .078-0.080 mark. I don’t have a precise way to measure it, so I eyeballed it a couple of times and that is what I came up with.
As to factory gas ports: I have also heard the 0.090 Colt Diameter, and I had contacted Daniel Defense in the spring about the 10.3" barrel diameter and they told me it was in the 0.080-0.085 realm.
I don’t have access to a vise where I live, but this weekend I will be traveling to my in-laws and they have one. I will install the barrel and DD mk18 RIS II then. Here’s a pic of the current set-up.
My phone can’t capture the barrel markings, but they read:“5.56 NATO 1/7 MP CHF CL PALMETTO” there is also an “87” stamped in front of the barrel extension.
This is some pretty OCD stuff, but I saw this on a forum somewhere and thought it was interesting so I copied it. I didn’t keep the link so can’t attribute it to the original poster, unfortunately.
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The way I align gas blocs is to plug the bore with a plastic cap and put a plastic hose in the chamber. Then allow air to flow monitoring the air volume with an airflow gage. I just move the gas block around until it maximizes the airflow then clamp it. No measuring required.
There is another method using a 1/4" long dowel pin which is just smaller diamter than the hole in the gas block or the gas port in the barrel (which ever is smaller) For a 30 cal barrel put a 1/4" outside diameter piece of plastic tubing down the barrel past the gas port, then put the pin into the gas block though the gas port hole, then slide the gas block onto the barrel. when the gas port holes align the pin wll drop into the hole in the barrel, locking the gas port to the barrel. Feel that the pin isn’t binding and clamp the gas block. Then pull out the 1/4" plastic tube and the pin will drop into the bore. That only works if the bore is larger diameter than the thickness of barrel wall and the clear distance of the hlole into the gas block, That method won’t work on a bull barrel 17 Rem, but it’s quite simple and accurate for big bore barels.
Method 3. Use a borescope and look out of the gas port from the inside. Don’t use the internal illuminator, rather use a light shining into hole for the gas tube, with the gas tube removed. I suppose you could illuminate it though the gas port itself with a fiber optic cable attached to a bright light source (green laser?). I haven’t tried this method, but I have inspected gas ports from the inside and think it would work.
I use the air method as I have the fittings and gauge made up on my bench and it works with all barrels. You can actually do a decent job just by blowing though the muzzle end of the barrel with the chamber plugged (fired brass) and just listen to the hiss of the air flowing though the gas tube. The hiss changes pitch slightly as the two holes align.
I’'ve never attempted to align a gas block by measuring and using pencil or scribe marks. The entire point is to get the holes aligned. Getting the gas tube to pass though the barrel nut squarely is just a matter of tweaking the shape of the gas tube. I use a dowel pin which just passes though the receiver casting to align the barrel nut before attempting to put the gas tube though it.
It’s possble to make a real mess if you try to torque the barrel nut with a gas tube passing though it. How do I know that… sigh…
