This -><- close to choosing 6.8 SPC vs 6.5 grendel

[Todd.K]

...according to the data results from over 10,000 test shots fired by the US Military in conjunction with some of the most experienced wound ballistics experts, 6.5mm provided the best accuracy, but 7mm provided the most destruction to the target. 6.8mm was the middle ground between them and best average performer between 6mm, 6.5mm, 6.8mm, 7mm and 7.62mm projectiles tested. ...

I am not limited to, nor very interested in, using bullet designs that are considered legal for military use.

I’m sorry that none of the people who developed the 6.8 wildcat understood that a test barrel is made to absolute minimum, and then ammo is loaded to MAP and not “pressure signs”. Nothing was wrong with the SAAMI print.

The idea that there was/is something wrong or that the velocity missed some critical benchmark severely damaged the adoption and popularity, and still does.

[win&legend]

It's not so much an issue of diameter of but of SD of standard loads. Longer projectiles in general tend to have more CG shift on impact than short projectiles, thus there barrier blind performance tends to be inconsistent. If you made really heavy 6.8mm bullets that had a similar length to width ratio as typical 6.5mm projectiles, you would have the same issue. However the case designs facilitate more optimal use of different projectile shapes, so while we can play games with bullet weights for a given cartridge, there are limits. The aggregate of one may work better on average than the other.

Alternatively, as mentioned, I think a 100gr 6.5mm bullet would provide similar barrier blind performance to typical 110gr 6.8mm bullets (albeit giving up some post barrier terminal performance due to the lower mass while also ceding intermediate exterior ballistics), but it may solve the inconsistencies for barrier blind terminal performance. Small differences can have notable impacts. Don't assume because it "looks small" that the resulting difference is negligible as the info graphic indicates.

How do you explain the significant differences in KE transfer measured by load cells below the ballistics gel from Brass Fetcher's testing? More rapid transfer generally results in better soft target terminal performance. Tissue stretch of damaged tissue by the pressure wave that follows the projectile itself and it's fragments causes additional tearing, hence more rapid blood loss. It's the synergy of the two that results in the most destructive wounds. Clearly there is a difference in form factor and the role it plays in KE transfer rates (aka how much drag does it have on impact to the target).

Kinetic Energy Transfer - Penetration Depth Bare 10pct Gelatin.png

How about rod bearings in ICE's for example to illustrate how small difference can have substantial impact? The difference between 0.002" clearance and 0.004" in clearance (barely visible to a trained naked eye) is massive in terms of optimal oil viscosity required to generate the most robust hydrodynamic film. It is the load bearing capability of that film that keeps the bearings in service for more than a few seconds of operation before catastrophic failure and we are only talking 2 thousands of an inch.

[win&legend]

I am not limited to, nor very interested in, using bullet designs that are considered legal for military use.

I’m sorry that none of the people who developed the 6.8 wildcat understood that a test barrel is made to absolute minimum, and then ammo is loaded to MAP and not “pressure signs”. Nothing was wrong with the SAAMI print.

The idea that there was/is something wrong or that the velocity missed some critical benchmark severely damaged the adoption and popularity, and still does.

What does the information you have in quotes have do to do with adoption by the US Military or the achievable velocity? No where in any of the material I have presented did I indicate the SPC drawing error resulted in rejection by the US Military. And I would not call a chamber spec used by Daniels Defense, Wilson Combat, LWRC and Barrett a wildcat, nor would I dismiss the actual problem with how PTG dimensioned the original SPC reamers.

I would also argue that none of these manufacturers really care about SAAMI specifications which are a guideline for civilian fire arms, however manufacturers of mass production ammunition such as Hornady etc. do adhere to them for safety and liability reasons. I really don't care about the past teething issues, I care about where things are now and what's available now.

This thread's purpose was to analyze the two cartridges and to make a distinction between them so the OP could make the most informed choice possible. I have looked at these two cartridges quite extensively and the information presented is what I have found to indicate one is more optimal for it's given application than the other. If you want to justify why what you have is the "best", have at it. Your welcome to provide all the validation you want proving what I have presented is invalid.

And I'm not sure what your statement about using or designing bullets legal for military use is aimed at. They are not limited either, had you bothered to actually read the material you would have known the following: "The Army’s Project Manager, Maneuver Ammunition Systems, assembled a team of experts to determine if there were any commercial off the shelf (COTS) 5.56-mm bullets or other calibers that were better than M855.".

[win&legend]

It's really interesting to note that the cartridge actually had legitimate AP loads produced for it. While this is not relevant for civilian uses, it does show the actual origins and purpose of this cartridge. Very high on the list of criteria is feed reliability and component reliability in concert with increased terminal performance. These things all matter. My information indicates the case design facilitates better feeding and bolt longevity. That may not matter for certain applications, however it is very high on my priority list for any small frame cartridge outside of the native 5.56 NATO.

Everyone can use or argue the merits of their own choices. Everyone probably thinks what they have is the "best" because it's what they have. I had no skin in the game, I actually changed over from the anemic 5.45x39mm cartridge in an AA piston AR I had gotten cheap way back when 7n6 was available. After cheap loads and parts dried up, I had to really dig beyond biased gun magazine publications and find what the US Military's research concluded and I went with that. This is why I make my argument for this cartridge, because of it's capabilities in all facets, system reliability, load availability, exterior ballistics, terminal performance, component suppliers etc. It's the sum total.

[Todd.K]

The 6.8 in development was a wildcat until it became SAAMI. The velocity difference between the development hand loads and SAAMI pressure tested loads is where accusations that SAAMI messed something up came from.

The intermediate project testing of calibers was based on the best performing military legal bullets, OTM. The 6.8 was the best compromise, not even the best terminal performer. And no, I don’t find that to be very relevant to bonded or all copper bullet performance.

[win&legend]

You can get bonded and all copper bullets in ANY of these calibers. The military tested more than just OMT. 62gr AKT bonded SP was one of the 5.56 test loads and the second best performer through auto glass at 100 meters. They tested bonded TOTM's, Bonded SP's, OTM's, FMJs and OTM's. I'm not sure your conclusion that only OTM loads were tested is valid or that idea some how automatically disproves the aggregate performance differences between cartridges and calibers....the reports even talk about SOST projectiles like MK318's etc. But even if only OTM loads were tested, your comparing OTM to OTM. Not OTM in one caliber to a SCHP or bonded SP in another.

Trends tend to form with volume and consistency, so if you test a SCHP in one caliber against a Bonded SP in another, it's not a direct corollary of aggregate performance. However when you compare for example Hornady's factory SST loads in both calibers, the differences then come down to achievable muzzle velocity at given barrel lengths (typically limited by cartridge design and bullet weight) and how the bullet shape affects actual terminal performance, because both bullets are of the same construction and material. Typically, bigger bullets do more damage while high BC centric bullets have flatter trajectories and are a little more accurate in the field (less drift) because they are easier to shoot under less than ideal conditions (when two cartridges have similar MV's). The question becomes, which ones produce the best compromise at actual usable ranges under field conditions.

I think the Texas Hog Hunter provides some good insight from an anecdotal experience, but one that carries a lot of weight given that he has shot thousands of hogs (very literally that's his job, as of the article on AR Build Junkie, 6,500 kills). You start to notice trends and averages that a typical hobbyist hunter might not notice taking a few dear a year. And to no one's surprise, .308 was his best performer for single shot performance, followed by 6.8 SPC and 7.62x39mm. 5.56 and .300 BO were very anemic, often having to hit them more than two times even as close as 50 yards. 6.5 Grendel when using 123gr SST performed close to 6.8 SPC with the 120gr SST, but Todd Huey seemed to think the 6.8 had better average performance on target and there were other system related issues with 6.5 Grendel that were an issue. It's a single data point, however it illustrates other characteristics that tend to favor one over another. How much that matters, is up to each individual to decide. Factors to consider are NOT just exterior ballistics, but terminal performance, system characteristics such as feed reliability, component choices / availability etc.

None of this data suggests 6.5 Grendel is a bad cartridge or substantially under powered or is completely unreliable. However when looking at the whole package, the data suggests that real world performance favors 6.8 SPC on average, both with terminal performance and with system reliability as that's been the focus of the cartridge since it's development, as a service cartridge. I don't care about shooting a piece of paper at 800 to 1000 yards. It's fun, but meaningless. I care about performance on target at ranges that any of these cartridges can actually be used effectively. That's around point blank to maybe 500 or 600 yards under more ideal conditions for a combat cartridge and for hunting game, at best the range either can achieve fragmentation or expansion (depend on the load), which is right around 400 yards with the better loads, maybe 450's for some really hot hand loads in either. If your hunting game, at longer ranges (or for larger game) your going to need more power and size in something like .308, .30-06, 7mm Magnum etc, but with it comes increases in cost, system weight / bulk and recoil.

Let's look at brass fetchers testing for another corroborating example (yet again, because we seem to fail to acknowledge the results), unrelated to the testing the military performed. If you think SCHP's are going to save the 6.5mm barrier blind performance or 5.56 barrier blind performance....think again. It mirrors the testing and evaluation by members of the AMU. Barnes TSX bullets were used for all three "intermediate" cartridges (55gr TSX for 5.56, 110gr TSX for 6.8 SPC and 120gr TSX for 6.5 Grendel) and fired only 10 feet from the muzzle giving you maximum velocity (the most ideal).

Kinetic Energy Transfer - Penetration Depth Bare 10pct Gelatin.png

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Kinetic Energy Transfer - Penetration Depth Car Door 10pct Gelatin.png

Notice the area under the curves and where that's at? 6.8mm was the best performer again, even with SCHP's by Barnes. I think it's best if I stop here or I'll keep this thread going longer than it should, I have provided a lot of information, testing and validation and am highly confident in my reasoning resulting from it. Your free to disagree and I'll leave it at that.

[Disciple]

It's not so much an issue of diameter of but of SD of standard loads. Longer projectiles in general tend to have more CG shift on impact than short projectiles, thus there barrier blind performance tends to be inconsistent. If you made really heavy 6.8mm bullets that had a similar length to width ratio as typical 6.5mm projectiles, you would have the same issue. However the case designs facilitate more optimal use of different projectile shapes, so while we can play games with bullet weights for a given cartridge, there are limits.

That's the way I see it too. The shorter Grendel case biases it toward long bullets but it still works with short ones. A longer case of the same diameter would be interesting.

[Todd.K]

I was talking about the testing that went into choosing 6.8 for the intermediate caliber. You are apparently discussing some other testing, because most of those bullets didn’t exist when the 6.8 was developed.

Some bullets are specifically designed for barrier blind performance at specific cartridge velocities, others are for hunting through a wide range of velocities and different cartridges. I would say that the 6.5 bullet is not purpose designed for the 6.5G or barrier testing based on the above graphs.

As I previously said in this thread: I prefer the 6.8 in large part because there are a lot of good purpose built hunting bullets for it, while the 6.5 customers are generally more concerned with external ballistics. This has more to do with the market, than the diameter difference.

[constructor]

From what I have read, the 6.5 Grendel out performs the .308 once you get past 600 yards. Flatter shooting and retains more energy than the .308 after 600. 0-600 the .308 is superior.

Only when you compare a 123gr Lapua match bullet to a common 149gr 308 fmj round. Compare a 155gr Lapua in a 308 to a Grendel with the same length barrel.

[constructor]

I am not limited to, nor very interested in, using bullet designs that are considered legal for military use.

I’m sorry that none of the people who developed the 6.8 wildcat understood that a test barrel is made to absolute minimum, and then ammo is loaded to MAP and not “pressure signs”. Nothing was wrong with the SAAMI print.

The idea that there was/is something wrong or that the velocity missed some critical benchmark severely damaged the adoption and popularity, and still does.

Yes there was. The SAAMI print had a .050" long freebore. The 115gr bullets Hornady made at the time were jammed into the lands. The "Extreme bullets SSA was loading were jammed into the lands. The thing I don't understand is why didn't Barrett test the ammo made by SSA before they showed up on base to demonstrate the 6.8 for the Army.
Chris Murray has said before the print turned in to SAAMI was an early design and not the same as the chamber used in the PRI test barrels and rifles first produced by MSTN. A mistake on a reamer print from PTG around 2006 caused the second problem. They transposed a number on the print at the mouth of the case made a 80 degree angle leading to the freebore when it was meant to be a 45 degree angle. That sharp corner shaved copper off the bullets when fired and all of those copper slivers welded together to form a copper ring, a restriction that resulted in the same as the bullets being jammed into the lands.
If you go back into the archives on arfcom in late 2006 you will find a whole conversation between Art Kalwas (SSA) and Randall Roush(AR15 barrels) talking about the mistake on the PTG print.