Cycling Issue

[Vartarg]

I've got several AR format rifles, but haven't had a Grendel until I bought an upper from PSA recently. I mated it with a Colt lower that has a Timney trigger, and took it to the range today. I left the Magnetospeed at home this morning, planning to just focus on breaking the rifle in and shooting for groups.

I'd bought a box of Hornady Black 123gr factory ammo, and reloaded 25 rounds with unfired Hornady brass. The reloads were 95gr Vmax, Rem 7 1/2 primers, and 30.8 to 31.6 grains of H335.

At a 100 yards, groups were lousy....only really decent one was 31.6gr and that was a shade over an inch.

The real issue was that it wouldn't cycle my reloads...but the factory rounds were fine.

Maybe it's a buffer or gas block issue, but I'm stumped about why it handles the Black 123's going around 2500 fps, and not my reloads which were certainly moving at least a couple hundred fps faster?

Your input appreciated.

George

[Bearcatrp]

What magazines are you using? I put a H2 buffer in mine plus switched from C products to E Lander mags. Fixed. Shoots fine now.

[Bigs28]

Barrel length? Gas system? Adjustable gas block?

[Vartarg]

What magazines are you using? I put a H2 buffer in mine plus switched from C products to E Lander mags. Fixed. Shoots fine now.

I used both a PRI and a ARStoner....both worked fine with the factory stuff....

[Vartarg]

Barrel length? Gas system? Adjustable gas block?

18" barrel, rifle length gas system, adjustable block....

[LRRPF52]

This is an easy one.

With 18"/RLGS, your 95gr V-MAX load doesn't have enough plug dwell for the powder type/charge weights/gas setting you're using.

You need to try opening up your adjustable gas all the way.

If that doesn't work, you need to switch to a slower powder.

If you put a suppressor on it, the action would most likely cycle.

[Kswhitetails]

^This guy^

[grayfox]

Just a word of caution V on the 95's...
My data shows me 31.2 of H335 is max, and I only go (went) to 30.7 for my 18", 2733 ft/s, getting warm. 30.4 for the XBR powder as well. again in the 2770's and warm load...
Might want to consider dialing it back a bit on those at least until you get it broken in.
Typical overpressure signs don't show on the Grendel until you're already over the line.

[Mikey7373]

[grayfox]

The short answer is, that it is a case with a lower designed Max operating pressure, about 52 ksi. Most brass for which the thumb rules apply is designed for ~60 ksi pressures.
Others on here can give a deeper, more technical explanation and hopefully they will chime in.

[Vartarg]

Just a word of caution V on the 95's...
My data shows me 31.2 of H335 is max, and I only go (went) to 30.7 for my 18", 2733 ft/s, getting warm. 30.4 for the XBR powder as well. again in the 2770's and warm load...
Might want to consider dialing it back a bit on those at least until you get it broken in.
Typical overpressure signs don't show on the Grendel until you're already over the line.

Thanks....

[BluntForceTrauma]

The short answer is, that it is a case with a lower designed Max operating pressure, about 52 ksi. Most brass for which the thumb rules apply is designed for ~60 ksi pressures.

Bill Alexander once told me the Grendel case is like any other modern standard brass case — it can withstand ~60k psi with the rest of 'em.

The limitation is not the brass case. The limitation is the bolt. Not the absolute strength of the bolt, but the longevity of the bolt, the fatigue strength, over the repeated hammering of many thousands of cycles.

Because any AR15 caliber with a case head larger than 5.56 but constrained to the AR15 bolt has a "weakened" bolt, the maximum average operating pressure of the 6.5 Grendel has been held to around 50,000 psi.

Bill was always engineering very conservatively, with an eye toward durability and reliabilty, and wasn't willing to chase muzzle velocity just to impress the gullible.

He let those beautiful sleek 6.5mm bullets chase the velocity for him. . . .

Muzzle Velocity versus actual Impact Velocity on target — it's not how you start that's important, but how you finish.

[bj139]

Not only is the bolt weakened because more steel has to be removed but the larger case head (larger area) increases the bolt thrust.

[LRRPF52]

At the risk of getting yelled at for asking a question that doesn’t pertain to the original question asked to start this thread.

I’ve seen on here several discussions that with the Grendel it doesn’t show over pressure signs until it’s over the line or until it’s running way to hot , but why is that ? Why does the Grendel not show signs of high pressure until it’s over the line ?

I’ve been reloading bolt guns for over 10 years and am always trying to learn, I will start reloading for my Grendel soon after breaking it in with factory ammo but again what is it about the Grendel that makes it not show over pressure signs until it’s over the line ?

It's also important to identify what excessive pressure signs are, which we did in the 6.5 Grendel Handbooks, as has been done in other reloading handbooks from major manufacturers.

Without a chronograph, you are really limited from seeing what you need to see. In gas guns, which require a certain degree of slop in the action to reliably chamber and extract, you can get false excess pressure signs from the abuse that a production-grade bolt face often exhibits, when sharp surfaces shear the brass under bolt rotation, and the treatment of the primer with sloppy firing pin aperture fit to the firing pin head.

Headspace doesn't allow for slip-fit brass like in a bolt gun, so we typically see more back-and-forth shoulder movement and brass growth/contraction in gas guns. With that sloppy headspace for reliable gas gun operation, combined with rapid bolt rotation during unlocking, a lot of little things pop up when just looking at the brass.

My first metric for detecting pressure is the chronograph and known, published loads cross-referenced with my chrono results. Then I start looking at the case head, primer, and other surfaces on the brass.

If I'm seeing book speeds that correlate with book charge weights with a known load in a chamber that allows cartridges to drop free with gravity (not jamming the lands), but am also seeing flattened primers in Grendel, for example, is it because I've reached 60,000psi, or because of gas gun headspace slop?

[bj139]

18" barrel, rifle length gas system, adjustable block....

I have a Ballistic Advantage 18" barrel that is rifle length gas and it does not cycle.
It should be mid length gas. People have drilled the gas hole larger to get it to cycle.
I haven't drilled mine yet. LRRPF52 made a suggestion on a post last year as to what the hole diameter should be.

Edit: Here is one post.

http://www.65grendel.com/forum/showthread.php?16760-18-quot-Barrel-Rifle-Length-Gas-System-(RLGS)-vs-MLGS&highlight=Ballistic+Advantage+18%26quot%3B%26quot%3B