Once Fired Case Stretch vs Gas Block Adjustment

[Growler]

After setting up a new adjustable gas block, an interesting pattern of case stretch was found. As I experimented to find a reliable gas setting, it appears that the case stretch increased with each increase of the adjustable gas block. I used a Hornady Headspace Comparator with its B350 bushing to measure case stretch vs the original unfired factory cases. The unfired cases were all within one thousandth of each other, but the once fired cases varied in this interesting way:

ShoulderStretchVsGas_cropped.jpg

On the first firing, the gas was set too low, and no cycle occurred. As expected, considerable force was needed to eject the case. I believe this matches the one observed case stretch of 0.010". For the next two rounds, I increased the gas. These short stroked, but not enough to eject. The chart also shows 2 occurrences of 0.011" stretch. Additional adjustments found a reliable setting about 2/3 of the clicks open for my JPGS-9D JP adjustable gas block.

The gas setting is reliable for this round, and very docile. I'm using a rifle length Geissele braided wire spring and standard rifle buffer, 20" monster barrel w/ rifle length gas, JP LMOS Stainless carrier, and monster bolt. The factory Hornady AG ammo has no ejector marks, swipes or issues. Runs great.

Has anyone here seen case stretch vary with gas setting? Could this be caused by the bolt trying to unlock while still under residual pressure?

[Sinclair]

After setting up a new adjustable gas block, an interesting pattern of case stretch was found. As I experimented to find a reliable gas setting, it appears that the case stretch increased with each increase of the adjustable gas block. I used a Hornady Headspace Comparator with its B350 bushing to measure case stretch vs the original unfired factory cases. The unfired cases were all within one thousandth of each other, but the once fired cases varied in this interesting way:


Has anyone here seen case stretch vary with gas setting? Could this be caused by the bolt trying to unlock while still under residual pressure?

This is why I shoot a CZ bolt rifle. Back in the day our M1 Garands would ingest anything from National Match to armor piercing Machine gun ammo without a hiccup. Now there are too many variables to get aligned with the stars.

[Klem]

Growler,

I have never heard this and to be fair, I haven't done any measuring while adjusting the gas to notice it.

If the only variable being changed is the gas then it sounds like the bolt is unlocking while there is still enough pressure to fire-form a case. If the ammo is the same (dimensions, bullet, primer and powder) and the headspace in the chamber is unchanged from the factory then the bolt needs to be moving backwards for those cases to grow. I am assuming the more gas you bleed the quicker/earlier the bolt moves.

I'm thinking the priority would be to find the best gas setting for reliable cycling and wear on the gun, and whatever the case stretch is is collateral.

[Growler]

I'm thinking the priority would be to find the best gas setting for reliable cycling and wear on the gun, and whatever the case stretch is is collateral.

[grayfox]

Important thing going forward is to not bump your case shoulder back by too much. The "stock" resizing die instructions have you bottom out the die on the case holder, which resets the shoulder back to pretty much where your cases started out, meaning a 0.012-0.013 stretch each firing. Instead, set the die up just a tad (precise engineering term - LOL!!!) to setback the shoulder about 0.004. Opinions differ, some say 0.003, I go for 0.004-0.005 for a single-barrel caliber (eg, not shooting several grendels). This will minimize case stretch on the subsequent firings but allow a consistent chambering.

[tdbru]

Growler,
i'll describe what mod's i've done to reduce a problem i thought about, and then mention why it might help you, though it might not too.

1. i have a BCA complete upper. 24" 1-8. has GI std. BCG weight wise. park'd finish. rife length gas tube. my 1st AR. was worried about filling the action with powder residue since i'd heard that the AR gets dirty in the action area and to keep it reliable you have to clean it meticulously (true or not, that's what i've heard). i was concerned with the longer barrel, that the pressure in the gas tube would exist longer and drive the bcg back faster than normal, uncovering the end of the gas tube earlier in the cycle while the gas pressure would be higher than normal in the tube and cause even more crud to fill the action. so, how to slow down the bcg. the upper did not come with an adj. gas block. well, how do i add mass, then, to the bcg? being a std GI bcg it is already at the heavy end of what's available. hmmm........ i found a "heavybuffer" made out of stainless that weighs 11oz. a little under 2x of normal buffer weight. well, that's part of the reciprocating mass so adding mass to that should delay the bcg movement, hoping to give more time for the gas tube pressure to drop before the bcg backs up far enough to uncover the end of the gas tube. hoping to keep the action interior cleaner longer.

2. if you're bcg is unlocking the bolt with still enough gas pressure in the chamber to fireform the brass as it is being extracted (i find this hard to believe, but if that's the working hypothesis, then) perhaps slowing down the bcg a while during the extraction cycle might reduce the extra stretched cases. since you are running a light weight BCG, and you have an adjustable gas block, perhaps try going with a regluar GI bcg and even also with a heavybuffer. see if the case will stay in the chamber longer for the minimum gas setting that will cycle the action. this should end up having a case extract with lower internal pressure. if your hypothesis is correct then the cases should exhibit less case stretch above 0.010" that your getting.

3. all this assumes that the brass is all uniform in thickness, dimensions, and heat treat, so that any variation in case length after firing is due to your hypothesis and not just the variations in the brass itself.

Brass is an ideal pick for a self contained cartridge case, because it's got enough springiness to contract off of the chamber walls after firing, making it easier to extract, and enough strength to form an effective pressure gasket at the high operating pressures without failing. my hypothesis would be that due to variations in case wall thickness and heat treat, the amount of contraction after peak pressure varies and this is what you're measuring. if you've adjusted that gas block to just cycle the action, then i would think that the chamber pressure is quite low, probably next to atmospheric pressure level at that point, once the bcg unlocks the bolt and begins to extract the case out of the chamber. but.... i could be way wrong on my assumptions too.

by the way, so far, the interior of my AR action is remarkably crud free. the only carbon i'm finding is in the bcg interior expansion chamber and the tail/backside of the bolt behind the gas rings. very clean everywhere else. rifle cycles softly to me, but never having an AR before and certainly not ever comparing a carbine or pistol length gas system to the setup i've got, my impression of a soft cycling action is open to reassessment upon trying a carbine gas system upper.
-tdbru

[Mad Charlie]

Important thing going forward is to not bump your case shoulder back by too much. The "stock" resizing die instructions have you bottom out the die on the case holder, which resets the shoulder back to pretty much where your cases started out, meaning a 0.012-0.013 stretch each firing. Instead, set the die up just a tad (precise engineering term - LOL!!!) to setback the shoulder about 0.004. Opinions differ, some say 0.003, I go for 0.004-0.005 for a single-barrel caliber (eg, not shooting several grendels). This will minimize case stretch on the subsequent firings but allow a consistent chambering.

I use the Redding Competition shell holder set, makes it easy to set up shoulder datum and still bump the die with the shell holder, removing the effects of somewhat loose or sloppy press linkage and enhancing sizing consistency.