6.5 Grendel Reload Pressure



  • I need some opinions on this reload. I am shooting two different suppressed Larue 18” uppers in 6.5 Grendel. I am getting marks on the cases, but I am not sure if it is pressure marks or sharp extractor edges. This is by far the most accurate load in these two uppers.

    Factory loaded 123 grain SST loads are not accurate at all with a spread of 3-4 inches. The Factory loads show the same type of extractor marks on the brass. I do not have pictures of the brass from the factory loads.

    This is my load. Hornady 123 grain SST, 28.4 grains of 8208 XBR, CCI 450 primers, new neck sized Starline brass, COL 2.24”

    My velocities
    28 grains average 2424
    28.2 grains average 2445
    28.4 grains average 2480

    These are pictures of the 28.4 load

    https://imgur.com/a/vZGTN



  • I’ve never fooled with a Grendel but have a fair amount of experience with 556, and 6.8. I’ve never seen brass look like that that wasn’t over pressured. Do you have an adjustable gas block? If so you can tune it down til it opens on an empty mag, this may help some. As bad as that brass looks though I think you have something over pressured.



  • @dogdr said:

    I need some opinions on this reload. I am shooting two different suppressed Larue 18” uppers in 6.5 Grendel. I am getting marks on the cases, but I am not sure if it is pressure marks or sharp extractor edges. This is by far the most accurate load in these two uppers.

    Factory loaded 123 grain SST loads are not accurate at all with a spread of 3-4 inches. The Factory loads show the same type of extractor marks on the brass. I do not have pictures of the brass from the factory loads.

    https://imgur.com/a/vZGTN

    Primers not flat; could be extractor/ejector? as you stated. I would expect to see flat primers with a ejector mark if over pressured.



  • I do not have an adjustable gas block as of yet. The brass is ejected almost perfect at 4 o’clock so I am not sure if I could reduce the gas any more and it still function correctly.



  • Have you tried a different brand of brass? Like stated above the primers aren’t flat but the brass is tore up bad especially that one piece on the bottom left corner of the picture.



  • The factory loaded Hornady brass actually looks worse than the Starline brass pictures I posted. Reloads with once fired Hornady brass also has the ejector signs on the brass.



  • There is significant ejector wipe on all of those cases. Either that brass is bad/soft... or those loads are quite hot.

    No question about it.

    While the primers are not flat, I can tell there is a bit of cratering. That cratering isn't severe enough to be concerned with, but it is present. A couple of the primers look flat around the edge too.



  • Here are the pictures of my 28.0 grains of 8208 load.

    https://imgur.com/a/Biyp8



  • Here are the pictures from my 28.2 grains of 8208 XBR.

    https://imgur.com/a/gsGcC



  • Even at 28, still significant ejector ring/wipe. Something definitely not right with that brass. 28gr isn't super hot in most rifles with most components...

    Possibilities:
    Bad brass. Sounds like you tested other brass with similar results.
    So then that leads me toward bullets/powder/barrel.

    Any one of these could cause pressure at what would otherwise be a safe load.



  • @orkan
    What about a bad carbon ring build up? If his loads are warm to begin with and he’s developed a significant carbon ring think that could be causing these pressure signs? Just a thought



  • @bull81 said:

    @orkan
    What about a bad carbon ring build up? If his loads are warm to begin with and he’s developed a significant carbon ring think that could be causing these pressure signs? Just a thought

    Absolutely. There are frankly, limitless possibilities as to what is causing this. It could be a gas timing issue, in that the bolt could be trying to unlock prematurely. It's impossible to speculate. I just listed a few of the major culprits.

    An overly fouled barrel, in any capacity whether it be in the throat or throughout, can cause significant pressure to appear where none existed before.



  • I am not disagreeing with any of the observations and extremely thankful for the assistance. I want to keep my velocities near the 2490-2500 FPS Range so hopefully I can figure out the high pressure problems.

    Could this be due to a dirty chamber even though it is an AR15? Could any errant lubricant be causing the high pressures? If that is a possibility what would be a good cleaner that will clean the chamber without leaving any lubricant residue?

    I am wondering why the factory Hornady 123 Grain SST is also producing the same or worse extractor marks. Am I wrong to think that the factory loads should be loaded on the milder side?

    One question would be concerning COL. Published data suggests 2.25 to 2.26”. I did load these to 2.24” to load better in my Elander magazines so that the tips would not be touching the front inside edge of the magazines. That is another problem with the magazines COL allowance I have to address. Would the shorter COL cause such a spike in pressure?

    Finally, do my velocities being in the same range as published data suggest that my high pressure might be caused by something other than too much powder?



  • Yes a dirty chamber or any oil, or other foreign substance can cause pressure problems. Give it a good cleaning then dry patch out the barrel and chamber. You shouldn’t have much lube on the bolt and carrier. Just a dab at the bolt cam, and gas rings, and a touch of grease on the lugs.
    I wouldn’t use published velocities as a standard to base anything off of, some barrels are faster than others, the barrel length may be different for the published velocities, and some guns run into pressure faster than others. Somebody a lot more knowledgeable than me may can explain why.



  • @dogdr said:

    Could this be due to a dirty chamber even though it is an AR15?

    Yes, could certainly be.

    @dogdr said:

    Could any errant lubricant be causing the high pressures?

    Yup, could certainly be.

    @dogdr said:

    I am wondering why the factory Hornady 123 Grain SST is also producing the same or worse extractor marks. Am I wrong to think that the factory loads should be loaded on the milder side?

    Hornady is known for putting out VERY hot ammo. I have had more students show high pressure with hornady ammo than any other brand.

    @dogdr said:

    Would the shorter COL cause such a spike in pressure?

    Could, but I wouldn't call it a spike. You need to do a proper pressure ladder, starting low and working your way up to determine this.

    @dogdr said:

    Finally, do my velocities being in the same range as published data suggest that my high pressure might be caused by something other than too much powder?

    The published data is typically with 26" barrels, in bolt guns. Your rifle is an 18" AR which should be running significantly slower.

    Two articles for your consideration:
    https://www.primalrights.com/library/articles/understanding-pressure
    https://www.primalrights.com/library/articles/load-development



  • One question that I have been unable to answer completely is what effect does a suppressor have on pressure. I am using a Silencerco Hybrid Suppressor on both of the Larue uppers.

    Will a suppressor cause an increase in pressure? If so how much do you have to decrease the load to equal shooting without a suppressor? If a safe load without a suppressor is 28.5 do you have to go down to 27.5 to get a safe load?

    Or could the extractor swipes and abnormal primer signs be caused by early opening of the bolt due to extra blowback gas from the suppressor? The uppers eject brass around 4 o’clock so I am not sure the gun is overgassed by that observance.



  • A suppressor does not drastically increase "pressure" as it pertains to a bolt gun. In a bolt gun, the brass maintains a sealed chamber through the entire firing event. However a suppressor will drastically change the operation of gas operated a semi-auto. Not so much due to what goes on before the bullet gets to it, but after.

    The back pressure caused by the delayed "trapping" of gas in the suppressor will create more pressure at the gas port and typically advance timing. As the bolt unlocks and begins extraction of the case, the "plug" is removed from the chamber end, allowing gas to come back past the case and into the action. This is why you see gas guns with suppressors get 300 rounds worth of crud in the action when firing 30 rounds. This coats the chamber with crud, which can also adversely affect pressure/timing.

    Piston driven uppers of the appropriate design mitigate this largely, and when timed correctly will produce clean cases and a clean action. This is an extremely complex dance of interior ballistics and while I have a strong background in the subject matter, I have made a personal choice to avoid semi-auto's in the world of precision. I do not consider myself an expert on them. Keep that in mind when reading my responses.



  • @orkan
    While your modesty and honesty is appreciated you certainly sound like a expert on this subject :). You do a very good job putting into words what some of us know but can’t explain, and for those of us that don’t know you do a good job of explaining clearly. That’s a powerful ability that’s greatly appreciated here on this forum fella.


 

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