New AR build will not cycle open

*MALICE* said:
Ya, so to the center of the hole (assuming .100" for the hole) puts it at .250" from the end. Which when using a .025" gap matches the .275" distance of the gas port from shoulder ;)
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That wouldn’t work, let’s work backwards. After scouring AR-15.com, I’ve found that the minimum gas port for a rifle length gas system is .093”, so let’s work with that number.

The gas block to back edge of the gas hole would be .300 (.200” + .100” gas hole diameter)

Back edge of gas port is .368” (.275 to gas port + .093 gas port diameter)

So if you leave .025” gap you would still be covering .043” of the gas port. (.368”-.300-.025”)

So what is the correct gap?

The centre of gas block hole to edge of gas block is .250” (.2 to edge of gas port and gas port diameter divided in half)

The gas port centre to barrel shoulder is .3215” (.275” to edge of GP and gas port radius is .0465)

.250”-.3215=.0715” gap you need.
 
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griffin86 said:
That wouldn’t work, let’s work backwards. After scouring AR-15.com, I’ve found that the minimum gas port for a rifle length gas system is .093”, so let’s work with that number.

The gas block to back edge of the gas hole would be .300 (.200” + .100” gas hole diameter) 1

Back edge of gas port is .368” (.275 to gas port + .093 gas port diameter)

So if you leave .025” gap you would still be covering .043” of the gas port. (.368”-.300-.025”)

So what is the correct gap?

The centre of gas block hole to edge of gas block is .250” (.2 to edge of gas port and gas port diameter divided in half) 2

The gas port centre to barrel shoulder is .3215” (.275” to edge of GP and gas port radius is .0465) 3

.250”-.3215=.0715” gap you need.
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Ok, you have some numbers wrong....or where you're measuring them from is off. We're going to use center-center distances.

1: The distance on the drawings (I double checked) for an ar15 barrel from the gas block shoulder to the center (NOT edge) of the gas port is .295" (I mistyped .275" earlier in the thread). often that number is rounded up to .300" so we'll use that.

2: Correct, .250" from edge to center, assuming that's actually the measurement... (I have a feeling it's a little longer than that but we'll go with that number for right now.)

3: see '1', you have this size incorrect :)

so .300"-.250"= .05" would be the hypothetical using the above numbers. But I thinks it's actually .300"-.275"=.025"

But I'm thinking the gas block hole position is actually closer to .275" (not to doubt you but I measure stuff for a living and I only trust my own measurements ;) ) and that puts the shim thickness at .02" which closely matches Superlative's instructions

Anyhooters, regardless of the numbers, the OP needs to check his gas block placement, then make sure he's adjusting the set screw properly before he goes any further :)
 
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Thanks again for all the input. I think I need to look at the gas block placement and adjustment. So if the adjuster is all the way in (closed) is that the same as having a regular gas block?
 
Helka said:
Thanks again for all the input. I think I need to look at the gas block placement and adjustment. So if the adjuster is all the way in (closed) is that the same as having a regular gas block?
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No. It would be like having no gas port (although in actuality you'd get some very small amount of gas flow, just not enough to cycle)
 
TomExAF said:
A lot of focus has been on the gas block. What about using a rifle buffer and spring? That may fix a timing issue (if not the gas block)
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Won't fit in a carbine buffer tube. He'd have to change his buffer tube and stock. Making sure his gas block is set up properly is the first step.
 
All of the above math and numbers seems to be over-complicating what you're experiencing. The diameter of the hole in the gas-block that aligns with the gas port is typically .125". Gas ports are located .295" with a tolerance measured from the hand-guard cap/gas block shoulder. There's also a tolerance on the alignment of the gas port in the barrel (looking at it from the top) side to side - meaning a gas port can be minutely 'off-centre' from what would appear to be the 'sides' of a barrel, again from the a top view.

With a gas block pushed tight against the shoulder, you should still be getting far more than enough gas to cycle the action. Double check your gas block settings given it's adjustability.

You may also want to check for mag-drag. Depending on the magazines used, the carrier assembly may be hanging up slightly on the follower - this typically occurs when the gas system is right on the cusp of functioning, but not quite there.
 
He already has a rifle length buffer tube. It is (not) matched with a carbine buffer (and spring?)
Either get a cabinet length buffer tube or go all rifle length.
 
*MALICE* said:
Ok, you have some numbers wrong....or where you're measuring them from is off. We're going to use center-center distances.

1: The distance on the drawings (I double checked) for an ar15 barrel from the gas block shoulder to the center (NOT edge) of the gas port is .295" (I mistyped .275" earlier in the thread). often that number is rounded up to .300" so we'll use that.

2: Correct, .250" from edge to center, assuming that's actually the measurement... (I have a feeling it's a little longer than that but we'll go with that number for right now.)

3: see '1', you have this size incorrect :)

so .300"-.250"= .045" would be the hypothetical using the above numbers. But I thinks it's actually .300"-.275"=.025"

But I'm thinking the gas block hole position is actually closer to .275" (not to doubt you but I measure stuff for a living and I only trust my own measurements ;) ) and that puts the shim thickness at .02" which closely matches Superlative's instructions

Anyhooters, regardless of the numbers, the OP needs to check his gas block placement, then make sure he's adjusting the set screw properly before he goes any further :)
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I was taking measurements from my barrel. I apologize, you are right and I’m wrong.
 
Helka said:
Ok. I will need to look at the instructions again.
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Once you ensure your block is positioned correctly, screw the adjustment screw all the way in. Go one click at a time out until the bolt locks open. When it does, maybe go one click more. Usually you want the brass to eject between 3-5 oclock.
 
TomExAF said:
He already has a rifle length buffer tube. It is (not) matched with a carbine buffer (and spring?)
Either get a cabinet length buffer tube or go all rifle length.
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His op says he's running an H1 (1tungsten, 2 steel weights) which means it's a carbine buffer/buffer tube.

You can't run a rifle buffer inn a carbine tube and vice versa.

He is running a rifle length gas tube/barrel, but that's got nothing to do with the carbine buffer system he's running
 
*MALICE* said:
His op says he's running an H1 (1tungsten, 2 steel weights) which means it's a carbine buffer/buffer tube.

You can't run a rifle buffer inn a carbine tube and vice versa
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Yes, that's exactly what I though I typed.
Sorry but I was sure I read that he was running a rifle length buffer tube. The H1 is the buffer...
 
Helka said:
Where would I look for signs of mag drag?
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Perhaps I worded my previous response poorly - rather than looking for mag-drag, you might be experiencing that in conjunction with the issues with the gas system. Rather - if the action is that close to cycling, but not quite enough to lock back on an empty magazine, additional friction of the magazine follower can exacerbate the problems overall.

It sounds like your rifle is nearly there in function.
 
griffin86 said:
It's either you don't have the gas block on right or your BCG is outrunning your magazine follower.
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This is the precise issue I encountered the first time I used an adjustable gas block on my stag 10. Granted we’re talking larger scale but the physics are still the same. I was 100% stuck on “not enough gas to cycle the bolt far enough back to hold open on the catch” and down I went over complicating the matter. Turns out all I had done was over gassed the gun and the BCG was moving far too quickly to hold open on an empty mag and wouldn’t even strip rounds out of the mags. You wouldn’t have encountered the second issue as you mentioned you were only single feeding rounds so you would have never had the bolt failing to strip rounds out of the mag.

My money is on this, as I have personally never used special tools to align a gas block, or brought out the feeler gauges to perfectly align gas blocks and I’ve never had cycling issues with the 10+ rifles I’ve assembled over the years. I don’t know the procedure you followed when adjusting the SA gas block (same one I have on my stag) but I found it to be very temperamental. One click adjustment per round until it was set just right.

I’ll also point out that a second trip to the range is likely going to be your most cost effective option so far.
 
*MALICE* said:
My opinion (ymmv), a lot of the other gas block designs are either: a) not click adjustable, b) they don't have a locking mechanism to prevent the adjustment screw from moving once set, c) if they do have a locking screw, it's in an inconvenient location for quick adjustments

Superlative checks all those boxes, but also comes with a feature that I'd probably never use *shrugs*

Which blocks do you have in mind as 'next on the list'?
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I am running SLR Riflework's Sentry's currently and am quite happy with them so far. The adjustment screw is right on the front and has a heat resistant spring loaded detent. It is also available in steel or titanium and both are very lightweight(Ti more so of course).

There are other AGB's with these features as well. Here is a few...

Odinworks Adjustable

Recluse Tactical Adjustable Gas Block

Seekins Precision Select

Noveske Switchblock

POF Dictator

None of these have set screws though as it is not common to have a set screw in conjunction with a detent(nor needed imo).
 
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