Grendel ballistics overstated?

[ibgp3]

From what Tony is saying we should be pitching the 6.5 GRR to the Israeli's.

[ibgp3]

Specialized uppers could be in the field in 5.56 very quickly.
...but would there be any reason for specialized uppers in 5.56?

....no comment?

[JASmith]

The rationale for specialized uppers in the 5.56 is the 150-250 ft/sec boost in muzzle velocity one gets by going from 14.5" to 20-24" barrels.

That difference affects the range where terminal ballistics start to suffer.

With the Grendel, it probably wouldn't matter since the terminal ballistics are fine out to beyond where the chances of hitting (300-400) moot the terminal ballistics discussions for a carbine.

[Gunwriter]

I hate to burst everyone's bubble here, but those velocities are not overstated.
They are what I recorded from my 20 inch GDMR using factory ammunition.
It was shot side by side with the 16 inch gun.
So those velocities are accurate for that rifle................

David M. Fortier

[Gunwriter]

First: Yes, the ballistics stated were no doubt for a 24" barrel, and not a 20" barrel. And don't let anybody say the 65G crowd is a bunch of lying, hyping S.O.B.s. We are hereby, with input from Mr. Alexander himself, making a public correction to that misinformation.

John

NOT a 24 inch barrel........NOT misinformation but what factory ammo clocked from my 20 inch gun
on a hot Kansas day on my range.

[Drifter]

I hate to burst everyone's bubble here, but those velocities are not overstated.
They are what I recorded from my 20 inch GDMR using factory ammunition.
It was shot side by side with the 16 inch gun.
So those velocities are accurate for that rifle................

David M. Fortier

NOT a 24 inch barrel........NOT misinformation but what factory ammo clocked from my 20 inch gun
on a hot Kansas day on my range.

Understood, but realize that those velocities are a little higher than what most of us experience from ~20" barrels. Of course, variables such as weather and chronographs can be a factor.

On another note, enjoyed your Grendel article in the latest issue of Book of the AR-15. While most Grendel enthusiasts are likely already in the know, it's good to see the latest information shared in print with a broader audience. Nice work.

[stokesrj]

I hate to burst everyone's bubble here, but those velocities are not overstated.
They are what I recorded from my 20 inch GDMR using factory ammunition.
It was shot side by side with the 16 inch gun.
So those velocities are accurate for that rifle................

David M. Fortier

It seems to me that people generally expect velocities to vary between rifles as a function of barrel length only. This simply isn't true.

To illustrate this I'll convey observations made by Troy Lawton and shared with me at the Army Marksmanship Unit at Fort Benning Georgia. Troy is a civilian but his job is to select the components that the AMU will use in competition for their service rifles, match rifles, and long range rifles.
Troy was describing the selection criteria for a new lot of Sierra 77 grain Match King Bullets for use in their service rifles for short range, 200 and 300 yards. Once he qualifies the component lots they are then sent to Atlanta Arms to be loaded. This test is conducted with ten new Krieger 20" 1:7.7 barrels made from consecutive blanks and using the same chamber reamer. They are fired from a machine rest which clamps the upper by the float tube and each of the ten uppers fire ten shots. (correction, that is 20 shots because that is how many rounds are in a regional match stage)
The acceptance criteria is one MOA at 300 yards for all 200 rounds. But the interesting thing and the point is that these ten consecutive barrels firing carefully loaded match ammunition will register velocity variations of up to 200 fps with as nearly identical set ups as is practically possible. Given this variance, how is it we think we can definitely tell if a velocity is from a 20" or 24" barrel? On average you can see the difference, but individual barrels could easily overlap.

[BjornF16]

Bob, that is an interesting story. I noticed that I get consistently higher velocities with factory and hand loaded ammo from my 18" Satern Spartan than I did with my 20" AA Entry...I originally attributed it to 5 groove rifling versus 6...

[Bill Alexander]

I suppose we should consider a bigger picture here. The Grendel case is specified to work at an internal pressure of 58,000 psi. This is a function of the web weight and also the walls. Janne at Lapua performed this work and may have been ahead of the whole development curve. It makes infinite sense to look at this pressure as it is a sweet spot for the use of smokeless powders in metallic cases (note the original 338 Lapua spec) We are constrained by long term durability of the bolt (and engineering cowardice) so the MAOP is set at 50,000 psi. A weapon such as the SCAR 16 would allow this constraint to be easily raised. However this is purely in the context of the military. In the world where most civilians use the AR platform it would be folly to raise pressures. We have to consider that there are a variety of barrels used and chambered for this caliber and with this variation in internals such as bore area will exist. The Grendel was designed from the outset to consider the heavier European rifling patterns and not create a dangerous situation in such barrels.

In stating any performance parameters it has become the convention to be conservative and to look not to individual measurements but the fleet results. It is quite usual for any individual rifle to report performance better than that stated. Again is it best for a person without a calibrated chrono and perhaps measuring a value not corrected for distance to see a figure well below what he was led to believe.

So returning to the OP the performance he encountered is slightly overstated but not lala land. In the context of military use, as long as the parent platform is not a conventional AR (we have better bolt configurations that negate the use of exotic alloys), then the performance may be raised within the current case and round OAL, which in turn does not place the weapon system on an upward weight spiral. Equally the performance upgrade brings the round into a more balanced state with respect to internal ballistics.

[JASmith]

...the interesting thing and the point is that these ten consecutive barrels firing carefully loaded match ammunition will register velocity variations of up to 200 fps with as nearly identical set ups as is practically possible. Given this variance, how is it we think we can definitely tell if a velocity is from a 20" or 24" barrel? On average you can see the difference, but individual barrels could easily overlap.

That's an interesting observation -- it also works out to a 7% variation in velocity from barrel to barrel.

The standard deviation of velocity from factory ammunition has been reported to be as high as 35 ft/sec -- admittedly a statistic from a few years back, but an indicator. Couple that with a reasonable spread in barrel performance, and one can see where an extreme spread approaching 200 ft/sec may be possible. This means that the highest is 100 ft/sec above the average.

A figure 200 ft/sec greater than the average of velocities reported by a number of folks using a variety of rifles and chambers appears to be outside what one would view as within the statistical population. (In other words, a statistician or quality control officer might use plain English words to the effect that something was likely wrong with the test...)

One wonders if there were one or two barrels that were way outside the pack and they skewed the statistics. Ten shots per barrel is approaching but still a long way from getting a good sample size.

Is there a possibility that Troy can give us a more complete picture of the tests?

• Average velocity for each barrel
• Extreme spread for each barrel

[ibgp3]

............ a person without a calibrated chrono .............

He is talking about me.
If I were told that my chrono was off by 5%, 7%, or even 10%, how could I be surprised.
...and for that matter, where do I get it calibrated?

[stokesrj]

Actually I typed that wrong it is 20 shots per barrel not ten. I accidentally reused ten because I had it in my head from the ten uppers. The reason they use 20 shots is that is the number of shots per stage in the Regional Match Course of Fire. Twenty shots slow fire standing at 200 yards, 20 shots rapid fire sitting at 200 yards and 20 shots rapid fire prone at 300 yards. There are also 20 shots slow fire prone at 600 yards but a different load is used there.

[JASmith]

The 20 shot per barrel does increase confidence in the average.

Nonetheless, it would be interesting to see more about the statistics -- it will help us all better understand how to use the numbers we see.

[Paul]

ibgt3 wrote "...we should be pitching the 6.5 GRR to the Israeli's."

And to the Austrians.

[Dogue]

A lot of excellent information in this thread!