6.5 Grendel Ammo Issues/Pressure Issues

[Flappy]

Hey all,
A few years back I bought a 16 inch 6.5 Grendel upper. Ended up not really shooting it much because of ammo and other stuff, and decided I would take it out and shoot it to make some brass this weekend, now that I have a press and dies and chrono etc. So I set up my target and chronograph and started putting together some groups at 100 yds. The rifle shot pretty well, but I ended up with two distinct groups, one .8 MOA and one .5 MOA in the same string of 15 shots, posted below. When I went to check the chrono data (ProChrono DLX, not the greatest I know but has been pretty reliable for me so far), the shots in the .8 MOA group were right at 2400 fps, but the five shots in the .5 MOA group were moving at 3000+ fps. I then took a look at the brass, and most were ok, but a few had bad pressure sign, and two had pierced primers. This is factory Hornady Black 123 gr ELD-M. I reached out to Hornady yesterday, they have yet to respond. Could this be anything but a bad ammo lot from Hornady? I've read about carbon rings in throats, bad chamber dimensions, fouling, etc. and want to know if I should scrap the barrel or just continue the RMA/warranty process with Hornady?

 

[Clint H]

Holy inconsistencies, Batman. That was all from the same box of factory ammo?

Other than carbon ring or some weird chamber/throat issue, afraid I don't have any ideas for you.

Out of curiosity, can you tell any difference in seating depth between the remaining rounds? If you have a sensitive enough scale, you might also be able to weigh and compare the rounds against each other to see if there is a powder charge difference.

Edit: or if Hornady doesn't want the ammo back, pull the bullets on a few and compare the powder charge and case capacity. I've seen inconsistent brass thickness do weird things before.

 

[Flappy]

That was the whole box, I didn't chrono the first five sighters at steel. But yeah one single box. Carbon ring is the only thing I have read that could maybe cause it but I have no clue what kind of velocity swing that would give, and I would think that once a ring is present, it would cause all the ammo after that to be high pressure, not the onesie-twosie type of inconsistency I saw. I couldn't tell a difference in seating depths visually when I pulled them out of the box, but I can't imagine even jamming to the lands would cause 700 fps ES/that much overpressure but I could easily be wrong...

 

[Clint H]

I've heard of carbon rings causing erratic fluctuations in velocities, so the up/down values you're seeing could still fit; however, I agree, 700fps seems odd. Anecdotally, I've heard of +/-200fps, but not 700 before.

No chance you have access to a borescope, do you? That'd probably be the surest way to tell.

 

[Flappy]

I don't have access to a borescope...I can try and post a picture of the chamber itself but it would just be with my phone. It also seems premature, because this rifle doesn't have more than maybe 200 rds through it, and if I really counted it probably is closer to the 100 mark.

 

[Clint H]

It can pretty hard to spot it from pictures of the chamber, sadly, and yes, that does seem like a pretty low round count to have one build up that badly.

You could try cleaning the throat with some C4, Slip 2000, carb cleaner, or CLP and a patch wrapped around a slightly oversized brush. See if you get any signs of excessive carbon residue.

Or just go for it, buy another box of ammo, and see if the new stuff also shows extreme velocity spreads. Life is short anyway.

 

[supercorndogs]

A carbon ring isnt going to spike velocity 600fps.

 

[LRRPF52]

Hey all,
A few years back I bought a 16 inch 6.5 Grendel upper. Ended up not really shooting it much because of ammo and other stuff, and decided I would take it out and shoot it to make some brass this weekend, now that I have a press and dies and chrono etc. So I set up my target and chronograph and started putting together some groups at 100 yds. The rifle shot pretty well, but I ended up with two distinct groups, one .8 MOA and one .5 MOA in the same string of 15 shots, posted below. When I went to check the chrono data (ProChrono DLX, not the greatest I know but has been pretty reliable for me so far), the shots in the .8 MOA group were right at 2400 fps, but the five shots in the .5 MOA group were moving at 3000+ fps. I then took a look at the brass, and most were ok, but a few had bad pressure sign, and two had pierced primers. This is factory Hornady Black 123 gr ELD-M. I reached out to Hornady yesterday, they have yet to respond. Could this be anything but a bad ammo lot from Hornady? I've read about carbon rings in throats, bad chamber dimensions, fouling, etc. and want to know if I should scrap the barrel or just continue the RMA/warranty process with Hornady?

Who made the upper? What barrel?

I don’t think I’ve even pushed my 22" .260 Rem to 3000fps with a 123gr.

How far was the chronograph from the muzzle?

You would have to load pistol powder in a Grendel to get a 123gr to go 3000fps from a 16” barrel I think.

That’s max load for a 24” barrel 6.5mm cartridges that hold 45gr of powder.

It’s good to notify Hornady of the Lot # though and your results.

Something is way off, to put it lightly.

You should be in the 2440-2460fps region with a 16” and 123gr ELD-M.

Optical Chronographs are usually 10-15ft from the muzzle, specified by the manufacturer in the manual.

 

[Flappy]

I don't remember who the manufacturer is, I bought it off GB through a retailer, but I don't think I paid much for it so I would imagine Bear Creek or similar probably. Chrono was in that range, not that I measured it with a tape but 4 or five steps back, and even if the recorded velocity is off by some percentage, the pressure signs would tell the same story I think, right?

 

[msgriff]

I wonder if the wrong type Grendel bolt for the chamber could be contributing to your issues, considering it's a used firearm of unknown origin.

 

[LRRPF52]

I wonder if the wrong type Grendel bolt for the chamber could be contributing to your issues, considering it's a used firearm of unknown origin.

The wrong bolt isn’t going to generate 3000fps with a 123gr from a 16” pipe.

That takes some seriously-fast burning powder.

 

[Flappy]

It is a Grendel Type II bolt...I don't think it was used but I have since trashed the email from the company

 

[supercorndogs]

You have any more of the same ammo? Weigh it.

 

[Flappy]

I don't have any more. Double charges seem unlikely, I suppose the powder swap seems most likely to me right now. Grendel has a case capacity of 2.3 CC, and from my limited understanding a double would well overflow that, if 30~ish grains gets 2500, and 30 gr is 2.xx CC already with common grendel powders. Unless someone with quick load can plug in what the velocity would be out of 34 or so gr that would be max capacity. The box I was shooting listed 2580 MV, so that means I'm losing ~15 fps/inch of barrel length which also seems inconsistent with what most say I should be losing per inch. Another reason to think fast powder where it shouldn't have been?

 

[supercorndogs]

I seriously doubt your cases would still have a primer in them if it was really 123g bullet going 3000fps. I also doubt it would eject the case vs locking up the action.

Grendels tend to mark up case heads.

A double charge isn't going to fit in the case.

Peirced primers could be the unknown bargin bun upper. That is the first place I would look.

That brass looks like two diffrent vintages of Hornady brass.

 

[Flappy]

The rounds came out of the same box that I bough at LGS a while back, so while I see the difference you're pointing out, I don't have an explanation for it. I did manage to find my data on the gun, I bought it from a GB store called KM Tactical as new, for about 300 bucks, with the BCG. I still think the upper itself is BCA, although they have no label on the website, the prices match and BCA was the only other manufacturer making a gov't profile Grendel in that price range. Carbine gas. In some of the other reading I've been doing...either the Grendel has a set back bolt face OR a shortened firing pin. I don't have my calipers handy, but I will measure both next time I get the chance. From what I understand, the standard firing pin treatment can cause pierced primers, although that still doesn't explain the velocity difference, or the presence of two distinct groups forming, very separate from one another.

 

[Cascade Hemi]

Your optical chronograph is probably picking up the muzzle blast. Move it further from the barrel and reshoot. Clean the barrel first too.

 

[Snuby642]

Make your own ammo.

Last time I had similar happen with a round I was 0.015 thousands in the lands. Conflicting load data.

Popping primers over normal speeds.