Alright, I have a new build that is short stroking.  I have pretty good reason to believe the gas block is the issue. After shooting and later taking the hand guard off I observed fouling along the top of the barrel right under where the gas tube enters the block.  It had a consistent "spray pattern" that went towards the receiver.   My gas block is clearly leaking.

Now, I understand that sometimes gas blocks need accumulate some build up and sort of "seal."  How many rounds does this take?  I only put 60 rounds (2 30 round mags) through my new build.

The 2 magazines I used are plain USGI mags that have always worked in every other AR I have.  I'm not 100% on the ammo but I believe it was 55gr PMC bronze.  The first 1/2 of the magazine went without a hitch (and accurate.)  Then I had a failure to eject.  The case looked like it had tried to eject but the bolt came back forward to soon and jammed it between the bolt face and the front of the ejection port.

Cleared it and shot the rest of the magazine without issue.

The entire 2nd magazine would shoot once or twice followed by either failure to feed (completely miss the round) or failure to eject.  At one point I took 2 rounds out of the second mag and put them in the first.  I shot this magazine (had 1 FTEject) and noticed the bolt was locking open on the magazine follower.  Well actually 2 failures to eject but the 2nd round kinda "dribbled" out of the port instead of getting caught by the BCG as it returned.

At the end I noticed the BCG and inside the upper was super filthy.  It also wasn't lubed the greatest.  It wasn't dry but not "wet."

I did not pick up any of the ejected brass.  The brass consistently ejected into a nice little pile about 2ft away from me at my 3 o clock position.

Once home I cleaned and lubed the rifle (more so than before) and double checked my gas block alignment (good to go.)

So, On my next outing I am going to take my SHTF M4gery(dead reliable) and swap a few things around (uppers & lower, Carrier.) and see if the issue persists.  I will also buy some quality factory loads. The rifle is going to be nice and lubed.  And I'm going to shoot the hell out of it.

If things don't straighten out I am blaming my gas block.  Its a cheapo that I bought thinking "any .750 gas block will do."  I'm thinking I was wrong.

Rifle Specs:

Upper:
Anderson slick side upper
PSA NiB BCG
GunTec hand guard
Ballistic Advantage 18" .223 Wylde SPR Stainless Rifle Length Barrel Premium Series
CMMG Rifle gas tube
ERGO Lo-Pro gas block (weak link I think)

The lower is a Ruger AR556 lower with a CMMG LPK and Ruger Elite 452 trigger.  I am using an A2 stock assembly.

So yea, I would like to know what the I.D. of a gas block for a .750 journal should be?  With my current gas block it has quite a bit of slop when the set screws are fully backed out.  I'm thinking I need something that has a tighter fit than the ERGO.

If on my next outing things don't change, the block is coming off, getting measured and more than likely replaced regardless.  I would like some reference points if possible.  Worst case I will go to my favorite gun shop and have them measure some gas blocks for me.

On the bright side, other than the short stroking; This rifle is amazingly accurate and a blast to shoot.

Thanks for the help!

UPDATE 3/26/17

Finally had the chance to go shoot this rifle again today and everything worked 100%

The only thing I changed from before was the gas block alignment.  The first time I installed it I butted it right up against the shoulder.  Now I have read different things about whether you should do that or not.  When I was adjusting the gas block as mentioned in my first post I kept a fingernails worth of space between the block and shoulder.  I also cleaned and lubed the rifle.

Today I brought along some more PMC Bronze m193 and added in some PMC XTAC M885 as well as some Federal 60gr Vital-Shok.  As well as a pile of random rounds from my random bullet pile.

I shot one of each round thru 3 different magazines ( a 10, 20 and 30rnd) and everything locked back as it should.

From there I proceeded to shoot the remaining m193 without any issue.

Then I shot the remaining M885 without issue

I didn't  shoot anymore of the vital-shok (it's expensive)

At the end I mag dumped the 20rnd mag with 20 rounds out of my random bullet pile.  Again zero issue.

Total I only shot probably 75 rounds total thru this rifle today buy they all worked perfectly.  Unlike the first time...

So yea, all is very very good!

It will probably be something just over .750, but I don't know for sure.  Do you have a way to measure your barrel and the ID of the gas block?  You'll need a few thousandths to allow for a "transitional" fit that allows you to actually put it on without force, but not much more than that I'd guess.

ETA:  A google image search turned up a bunch of drawings, most of which show .750 nominal with +/- .001 or .002.

Yea I'm thinking the cheapo gas block has an extra thousandth or two.

My Ballistic Advantage/ Aero GB measures .7495 and my SLR GB measures .750.

I've seen .747 and .752 at the extreme ends.

Some gas blocks just have a difference in their ID.

I remember coming across a nitrided barrel and a BCM gas block and it was clear to me that it would not easily come off.

I like it when I need a torch to get a gas block on.

This ^^^^^^^

That is why I like the BA/ Aero GB's because they are tight.  The SLR are not bad either.  I won't use a GB that is loose and spins on the barrel easily.

And mine does.  It's getting replaced (and measured so i can send a complaint to ERGO.).

Fu*ked up gas block for sure. All of the gas blocks I've installed, .750, needed to be lubed (barrel) and lightly tapped with a rubber mallet.

0.7500" to 0.7507"  (0.7500 + 0.0007")

The barrel OD should be 0.7485" to 0.7495" (0.7490 +/- 0.0005")

EDIT:  It is quite easy to make a good clean, correctly sized, hole the size of a standard reammer (3/4 inch), and tools tend to wear in the direction of making a smaller hole.  I would measure the barrel as well, as these are more likely to be undersized.

It could also be the combination of 18" barrel with rifle length gas and the weak PMC bronze ammo

But it's weird it seemed to get worse the more you shot it. Almost like the gas block was moving, but you said it was fine when you took it home and cleaned it?

0.750" +/- 0.004"

These are precisely the numbers that you need. If the barrel O.D. were to be 0.7495"
& the gas block I.D. is 0.7500" it would be almost ideal for the average person & their
"feel". That much allowance will let you slide it on with one hand, provided that both
surfaces are perfectly clean & very lightly oiled. In my industry we run millions of
cycles each day in different molds in less allowances than that without oil or grease.

That's waaaaay too sloppy.

I was also thinking it could be the weak PMC ammo.  Before I go about totally replacing the block I'm going to try some hotter stuff.  But yes, when I got home and checked everything out; The gas block had not moved.  And there was a nice ring around the gas port on the barrel, where the port in the gas block was resting.

If I were to take the gas block and remove the set screws and set the barrel muzzle up on the table, and drop the gas block over the barrel; It would freely drop to the shoulder of the barrel.

Seriously, this thing has zero resistance when mounted on the barrel.

This is my first upper build and I didn't think anything of it when putting it together.  Now I'm thinking I bought a POS part. Fortunately it was dirt cheap (less than $20.)

Also I was brain storming at work today and got to thinking about how the problem got worse as I kept shooting.  Is it possible for "something" to have ended up in the gas tube?  Like small enough to get thru the barrel gas port but large enough to not get blown out the gas tube while shooting?  And subsequently accumulate some build-up and make things not work?

I'm adding take a pipe cleaner to the gas tube to the to-do-list.

Why?

+/-.004"? You'll have everything from "can't get it on with a hammer" to "can instal from across the room with a toss."

Strangely, the two worst gas block installs I've ever done were a Geissele block on an Aero \ BA barrel, and a very, very dubious "Red Barn Armory" (copy of the Seekins AGB, which seems to be the most copied adjustable block on the market) adjustable gas block on a Sig M400.

I don't mind them tight, but requiring a hammer to beat them into place shouldn't be necessary. And yeah, +\- 4 thou is a bit overkill

I want lego tolerances dammit.

Legos are a press fit.

Oddly enough, The Geissele GB's that I have measured .7505.  They were the largest out of the three different types that I have sitting around.  Granted this is measuring with a mid range digital caliper, it's not a $300 Starrett.

I would look towards a leaking gas key instead of worrying about the gas block.  Also, make sure the gas tube was made properly with the slight bulb on the end.

EDIT:

Seeing as ziarifleman cover the basics, I'm go into a little more detail.

You do not want a diametrical clearance of more than two thousandths of an inch (0.002"), mainly for aesthetic reasons, and the perception of a wobbly leaky gas block.

You cannot have a diametrical clearance less than half a thousandths (0.0005"), or you are going to have to use an arbor press to get it on, and after you do, you aren't going to be able to rotate it to clock it.

So, that is the range of diametrical clearance - 0.0005" to 0.002"

Now, it is really easy to drill and ream a hole to very tight tolerances, +/- 0.0005" the diameter of the reamer. further reamers come in standard sizes, your standard drill sizes and your standard fractions.  You can get 0.001" over or under sized reamers but they are slightly more expensive, so you don't design stuff to use them unless really necessary.  As cutting tools are used, they wear, so with drills and reamers, the size of the hole cut gets smaller.

So, when designing something with a close fit hole, you normally start with a hole size that is 0.0005" to 0.001" over a nominal standard drill/reamer size, in this case 3/4 inch (0.750"), and allow it to shrink (as the tooling wears) to the minimum allowable diameter, then you throw out the tool and get a new one.

Now, since all gas blocks have to fit all barrels, That means the largest barrel journal OD must stay 0.0005" smaller than the largest gas block ID in order to keep the half thousandths slip fit clearance, and, the largest barrel journal OD must stay within 0.002" of the smallest gas block ID to keep from having a sloppy fit.  Likewise, the smallest barrel journal OD must stay be 0.002" of the largest gas block ID, and, the smallest barrel journal OD cannot be less than 0.0005" of the smallest gas block ID.  

If you assume you start with the largest possible hole you get with a standard 0.7500" reamer which is 0.7505", then all the rest to the dimensions fall out, by applying the above logic.

Oh, and one last thing, it is very easy to drill a hole to close tolerances, I doubt you have a problem with the gas block being too big, as that would require a reamer that was significantly over sized, or really poor drilling practices, which would show up and a horrible surface finish.  If you do have a problem with a loose fit between the barrel and gas block, it more likely the barrel journal being to small, as that is the easier part to mess up and still have look fairly good.


A 0.002" slip fit will drop on without resistance.

No offense, but if this is your first build, I would look to workmanship issues, not material issues.

Lots of information there thank you!  And no offense at all.  I would not be surprised if I did something wrong.  But from what I can tell everything is done the way it should be.  I still need to get out and do some more testing though before I do anything.

Standard gas blocks are cheap.

Just change it and see what happens.

It's not a bad idea to have a few spare gas blocks and gas tubes.

BTW, it's a rare occurrence that I don't have to polish the inside of a gas block to get it to fit a rifle's barrel. After getting it to fit, I plug the breech end and blow air through the barrel just to see what sort of flow I get into the gas block. You can sometimes preempt a problem that way.

Update in OP

I doubt it was the diameter of the gas block to barrel but more the location of the gas port to the shoulder of the block. Mismatched gas holes will make much more of  a difference than a couple of thousands on the block OD.