Newbuild - Short Stroking & Gas Block Alignment

[Oso Polaris]

I convinced a friend that he should build a AR15 in 6.5Grendel...this would be his first big boy rifle. He dropped coin on all premium components, including a Precision Firearms barrel (18", rifle length gas), BCG and muzzle break. After waiting 7 months for it to arrive, he was in a hurry to get it built. I have built/assembled a dozen AR15's (5.56 and 300BO), but this was my first Grendel and also first to use a PF barrel. Everything was going together nice and easy until I got to the gas block, which was an SLR Sentry 7 (clamp on style). During the test fitting the gas block would not sit flush against the shoulder on the barrel. It was late and I was rushing to finish in order to make the 8am range meet up the next day. I completely overlooked the very small false shoulder, located in front of the primary shoulder, machined on the Precision Firearms barrel. I did a forum search and read some threads about the "credit card with gap (1/16") was not uncommon (remnant of military clam style handguards needing the extra space in front of primary shoulder). I was using the HB Industries' Gas Block Alignment Tool, plastic precut orange stick. These are made in a .22cal set and .30cal set, with a series of different diameters pegs/rods. I believe the only difference between the two caliber sets is the location of where the precut break-off line is placed to match the bore diameter so that it snaps off cleanly into 2 pieces when hit with a cleaning rod run down the barrel. I grabbed a orange peg off a .30cal set and trimmed a little of length off the side that sits in the barrel so that the precut break-off point aligned with height/bore diameter (.264). Nice and easy no measurements - if stick floats back in and out of gas block as rotate barrel upside down then everything is good.

We took rifle to the range and it would only cycle the bolt far enough to eject spent case, but not pick up the next round or hold open. We swapped out magazines and issue continued. We fired 30 rounds through it (single shot) doing barrel break-in and sighting in scope. The brass was gently thrown 3'-4' away at 4o'clock position. We had a problem - I observed a raised ridge on the cases just above the webbing with the brass below (base of case) this ridge angles upward and the brass above (case body) sloped away. My initially concerned was that this may be result of too generous a chamber. However, further inspection of cases showed that the "raised ridge" only circumnavigated 75% of case with the highest point of ridge aligning on side with the extractor, and it gradually faded as it wrapped around the case. I thought it might be a dwell time/gas issue, which KLEM was kind enough to confirm and suggest a couple of solutions. I'll post his response because it was really helpful and a great explanation on dwell time issues. This rifle has 18" barrel with rifle length gas system, which have been known to be a bit finicky... buddy should have gone with the intermediate gas length.

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Before messing with the gas block, which I felt was centered, I swapped out the buffer to a 3oz (lightest I had on hand in spare parts bin) and opened up the SLR adjustable gas block (7 clicks). Back to the range, the brass no longer showed the raised ridge! However, now the surface of the cases were rough - heavily scuffed up, and not the usual smooth and shinny. It looked like a surface friction interference as brass case sticks to chamber wall and gets dragged out during extraction.
Again, KLEM was spot on about it indicating a dwell time issue. By reducing the buffer weight, the BCG was now able to move the BCG rearward slightly easier/faster after shot... reducing the dwell time and pressure build. A step in the right direction, but still not enough...

I got home and stripped off the gas block. Now I clearly saw the secondary shoulder machined on the barrel. I pulled out my calipers and began measuring everything. This is the first time I have encountered an AR barrel in which the gas port was same size as the hole in gas block (0.125"). Usually the gas port is "oversized" to government standard so you are all but guaranteed alignment with "close enough". What I discovered was that (1) "IF" the gas alignment peg is undersized diameter for gas port and (2) "IF" the gas alignment peg has been cut short (.308-.264 = .044") then these two combine to make it possible to get a False Positive for alignment.

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In this instance the gas alignment peg was able to tilt on an angle (lean backwards) while the gas block and gas port were not perfectly aligned and still give a "positive". The original position of the gas block, which was butted up against the false shoulder, meant that 1/3 of the gas port was covered. This explains why the rifle wasn't getting enough gas to properly move the BCG after each shot resulting in an extended dwell time. Yes... I feel like an idiot. I know better and should have confirmed with calipers... and not simply relied on a piece of plastic. Great learning experience that I could have easily avoided... Hopefully, this helps the next guy avoid the issue.

Edit: This morning's range session was a success. Tested it with a couple of different types of factory ammo and had clean extraction and feeding, LRBO, etc. Gun shoots 1/2" with Hornady Black Box 123gr HPBT.

[Oso Polaris]

Klem - thanks again for patience and advise!

This is Klem's advise/explanation on "Lock-up Time":

"The case bulging is a worry but also points to the cause of the short stroking. It means the pressure is still too high when the case is being extracted. As the case is being dragged back out of the chamber it is no longer constrained by the walls and now free to expand like a balloon. The first part to be free as it exits the chamber is the web and that's why it is bulging there. As soon as the bullet exits the barrel pressure drops otherwise pressure will take the path of least resistance and blow those cases up in your face. Cases are thicker down at the web so will support more pressure but thinner towards the shoulders and mouth to expand and seal at the mouth. But even thick cases down at the web will expand if pulled out too early.

That is not a 'break in' issue, it is early unlocking and extracting that is causing your short stroking.

The bulge is not from the case firing early while not in battery. The gun is designed to prevent firing unless the bolt is rotated completely into battery. What is happening is the gun has been fired and the bolt is unlocking and pulling back early. This happens mostly in short AR barrels and gas systems. The original AR system designed by Stoner was based on a long 20" barrel with rifle length gas system. Over the years the barrels have become shorter and gas systems along with it. Gas is getting to the carrier sooner and unlocking before the pressure has fully dropped. This means cases are still gripping the walls of the chamber and being dragged out with a lot of friction and stress on the extractor. This extra friction means the carrier doesn't go all the way to the rear to lock back on last round. In your case the heavier buffer is robbing even more inertia and not even going back far enough to pick up a new round. If the pressure is too high while the case is being dragged out it also explains the bulging. There is your problem - early unlocking while the pressure is still too high.

The deal is, after the bullet has passed the gas port there must be enough dwell time for the gas system to energise and impart enough inertia to the carrier to do a full cycle. But, too much dwell time and the system remains pressurised for too long (which is your case). it's a balance which we have some control over with different weights of buffers, carriers, adjustable gas systems and recoil springs. A new gun tends to be a bit rough so that factors in as well (your gun also, but this is a lesser worry and it quickly wears smooth)."

[montana]

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By reducing the buffer weight, the BCG was now able to move the BCG rearward slightly easier/faster after shot... reducing the dwell time and pressure build. A step in the right direction, but still not enough...


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I don't like to interfere, but the dwell time is simply the amount of time that the bullet spends in the barrel after it has passed the gas port and before it exits the muzzle end of the barrel. Changing the buffer weight does not effect dwell time, but increases or decreases the velocity of the bolt carrier group. An adjustable gas block will change the pressure, "AKA force" applied to the BCG...

[Oso Polaris]

Fair comment - then perhaps Lock-up Time is better term for the amount of time that case stays in chamber after ignition. This timing issue was the problem, and may be the result of insufficient amount of gas to move BCG, too heavy of a buffer system (buffer weight &/or buffer spring), or some other mechanical interference delaying/restricting the BCG's rearward movement.