6.5 Grendel Pressures -- SAAMI acceptance

[mehavey]

Does anyone have a source for the actual SAAMI pressure standards just approved by the Institute? With the published load data both in Hornady 8th and the Radford/Alexander sheets...

http://www.alexanderarms.com/images/..._reloading.pdf

... it's got to be mid 60's. (In fact the Hornady/8th max load for the 129gr SST/Win748 calculates out to be as high as 74k, and the 123SMK load from Radford hits 77k. Others hover around 62/67k easily)

Any actual experience out there before I start playing with this thing?

[Unclenick]

No personal experience with it, but the hottest maximum load Hodgdon lists for it is 50,200 psi. QuickLOAD's database has the CIP maximum at 360 MPa, which converts to 52,214 psi. The cartridge is too new for the 1992 freebee spec you can get from SAAMI, which means you have to pay to get the information directly from them, though they might tell you over the phone. But from the two sources I just mentioned, it looks more like low to mid 50's.

[mehavey]

At the risk of my looking really dumb ... What am I doing/entering wrong below?
(This comes from the next-to-last load listed in Hornady 8th)



I must have made a made a major blunder somewhere in the entry data (as evidenced by just
the velocity if not the pressure.) 35.1gr H2O case volume comes from here:
http://bulletin.accurateshooter.com/...-first-report/




.

[steve4102]

My actual case capacity for the Grendel (AA Brass) is 36.0.
QL does not work very well in the Grendel with heavy bullets. Using 36.0gr my calculations are very close to published data with 95-107gr bullets. Anything heavier than 107gr and QL falls apart. Duno why?

[steve4102]

How does SAAMI determine pressure standards for an unknown like the Grendel. Do they take the word of the cartridge designers, in this case Bill Alexander and Arne Brennan, or do they actually test it themselves?

How was the operating pressures determined in the first place? 50K seems awfully low compared to other AR rounds like the 6.8 SPC (58K).

[Brian Pfleuger]

Mehavey,

Couple things....

I don't remember the exact description QL uses for the "weighting factor" but I do remember that it gives you some guidance for adjustment. A while back, I found a calculation that relates the "over bore" status of a cartridge by dividing the case capacity by the bore area. In this case, (35.1/.05372) to get 653.3. The source where I found the calculation suggested that anything starting at about 1000 was "over-bore". Obviously, this cartridge is not over-bore but an adjustment to less than .5 might be in order.

QL lists the length of that bullet at 1.300 and describes it as "SST Interlock". I'm not certain but I believe that "SST" and "Interlock" are mutually exclusive or at least not always synonymous. JBM here lists the Interlock as having a length of only 1.169 inches.


I doubt those changes will amount to over 20k PSI but it can't hurt.

[Unclenick]

The first thing I'm seeing is 107.8% case fill. Spherical propellants like 748 just don't compress as easily as stick powders, so I'm immediately suspicious the default case capacity is too small. It often is, as that errs on the side of safety. QuickLOAD does case fill percentage on the capacity of a resized case capacity, but once you get over 30,000 psi, it's peak pressure depends on the larger fireformed case capacity. We don't know what that was in the Hornady test gun.

Any cartridge that starts life formed as a wildcat is, from another case, it can have its case capacity change once case making goes commercial and it is no longer being formed from its parent case. I don't know how Hornady's cases compare to the original formed from 220 Russian, but I know their cases in .30-06 and a couple other chamberings are light, meaning they have increased powder capacity. This needs to be checked for the Hornady cases. I don't have their #8 book, but presume they used Hornady brass.

The Wikipedia say Hornady is responsible for submitting the cartridge to SAAMI for inclusion in standards. That means they are likely to be the manufacturer of record responsible for making proof and reference loads for the rest of the industry. They will also have been the ones who set the pressure based on their testing and their cases. So call Hornady (always worth it for the phone system's answering message) and talk to one of their techs to learn what the maximum PSI is. He may even be able to tell you how the capacity of their cases compares to the capacity of cases formed from 220 Russian.

I changed the case capacity and went from the default 35.00 grains water capacity to 38.22 grains to get a match with the Tac and N140 and N540 loads given by Alexander Arms with the 120 grain SMK. Some of the 106 grain Scenar loads came out OK then, too. In any event, you want to measure the overflow capacity of your own fired cases (fill level to the case mouth and with no meniscus; not literally overflowing).

Those loads on the Alexander list that did not match well mostly came in with just a couple of exceptions I reduced the burn rate of the powder by as much as 6%. Hodgdon claims ±3% is as tightly as they make different powder lots. That means QuickLOAD's particular test lot might have been at either extreme, so you want to check ±6% as a possible value for your own lot or the one someone else used in a test.

[mehavey]

I spoke with Bill Alexander and he confirms QL has a problem with the PPC/heavy bullet combinations. When things finally get here I'll double check the case volume, but don't think I'll find it too far from the listings Hornady and others give.

http://www.6mmbr.com/65grendel.html
http://bulletin.accurateshooter.com/...-first-report/

I'll run the recommended loads/chronograph things/see what expectations/adjustments pan out and report back.

[Unclenick]

I'm still interested to hear about the measured case capacity of the current production brass. If this turns out to be a QuickLOAD modeling issue, I'd like to know about it as I am occasionally in touch with Hartmut Broemel by email and I expect he would be able to comment on the issue. I think he's also posting on one of the forums some. I've forgotten which one, but I did run into it before, so you could ask him yourself in that venue.

This article includes some information by Arne Brennan on which powders he thought were best at the time he wrote it. Also that he used moly-coated bullets in his initial velocity objective successes. Also that Alexander loads to 49,000 psi. The idea is apparently long barrel life, and Brennan says the original barrel is still driving tacks after 6000 rounds, which is almost about twice what some .30 cal service rifle barrels get.

[steve4102]

I'm still interested to hear about the measured case capacity of the current production brass.

My actual case capacity in grains of H20 averages 36gr. This is fired AA (Lapua) Brass.

[PawPaw]

Originally Posted by peetzakilla

QL lists the length of that bullet at 1.300 and describes it as "SST Interlock". I'm not certain but I believe that "SST" and "Interlock" are mutually exclusive or at least not always synonymous. JBM here lists the Interlock as having a length of only 1.169 inches.

I used to believe the same thing, but I bought a box of Hornady bullets this year that was marked as Interlock SST. It seems that Hornady is putting the Interlock feature in SST bullets. From their product page:

Heavy construction with InterLock® feature delivers deep penetration every time.