Is 6.8 a viable alternative to 7.62 for an MBR?

[maximus83]

I'm not playing these calibers off against each other, just looking at it from the standpoint of a shooter who has multiple AR's in 5.56, bolt action rifles in 7.62, and wants to add a heavier caliber semiauto "MBR" type of battle rifle to the collection. Currently unable to afford both, but I'd like to start with an MBR for range shooting up to 600 yards, deer hunting (if I can keep it under 10 lbs), and of course defensive or "SHTF" operations.

I know the 6.8 does fine at deer hunting and range work, and the ballistics programs indicate it's good for paper punching up to 600 yards. But my questions are mostly on how the 6.8 compares to 7.62 as a battle round.

* Within that first 600 yards (which is probably the useful/practical range of the 6.8), how well does the 6.8 compare to the 7.62 at typical combat applications: CQB work, sniping, penetrating light cover, etc.? would the 7.62 likely be a lot more effective at things like penetration of cover within the first 300 yards? Are there any published studies by the military or anyone else comparing these rounds?

* What about beyond 600 yards? Obviously 7.62 NATO can still be effective out to 800 or even farther, depending on shooter, rifle, and glass. But is it possible to extend the effective range of 6.8 beyond 600 yards, if you have to?

* What do you think? If you had to pick 6.8 or 7.62 to start with as your MBR, given the pros and cons of both (a big pro of the 6.8, for me, is the fact it works on existing AR platforms and even my current lowers), which would it be?

[rob_s]

Not what you asked but...


Do you have a use for 600+ yard shooting? Have you ever done it before? Do you have a location where you can do it regularly?

Think about your practical applications for what you're looking to buy/build and the answers should present themselves to you in short order.

[Failure2Stop]

6.8 is a viable combat cartridge.

You seem to be fixated on "Assault" rifle versus "Battle" rifle terminology. As far as that goes the 6.8 is a very good assault rifle cartridge. That being said, the definition of a system is far less important than the individual employing said rifle. When someone defines the battle rifle as something that is effective at X distance, you must remember how that was achieved- mass volume fire. That employment technique is quite different than individual target engagement. There is a reason that no army in the world carries a "battle" rifle as the primary armament.

Maybe DocGKR can come in and get into the nitty-gritty.

[maximus83]

Thanks for the replies. It's probably simpler to just remove my usage from the picture, and phrase the question I'm looking at in more general terms:

Does anyone have information (published content, first-hand experience, ballistics studies, whatever) on how effective the 6.8 is, compared to 7.62, as a combat cartridge used in the following scenarios:

* CQB
* Penetration of light cover (not really a separate scenario, but a factor that may apply in any of these other scenarios.
* Sniper/DMR use (at 600 yards or less, given that is often considered the practical effective range of the 6.8)
* Firefights with high round-counts (whether firing FA, select fire, or rapid-fire semiauto)


I've found the excellent articles on the 6.8 itself, including Zak Smith's articles, and some stuff by DocGkr on terminal effects of the 6.8 at Tactical Forums. But I haven't been able to find much specific evaluation of how the 6.8 stacks up to the 7.62 as a combat cartridge, focusing on the effectiveness of the two rounds (and NOT looking at peripheral advantages such as lighter recoil for the 6.8, even though such advantages are useful). Obviously a problem here is, only a few units of the military have allegedly evaluated it, and I don't know if that info is even available anywhere. In other words, there just may not be that much info out there yet, on the above questions.

[DocGKR]

Have you read this: http://www.dtic.mil/ndia/2008Intl/Roberts.pdf?

[Ed L.]

Thanks, DRGKR,

I never get tired of reading that presentation.

[maximus83]

+1. Some interesting info, thanks for posting it.

One question came up in reading it: What is "fleet yaw"?

[DocGKR]

Fleet Yaw is the terminal performance AOA variation caused by inherent variability in each rifle; fleet Yaw is caused by weapon to weapon variations separate from projectile induced AOA issues. 5.56 mm FMJ had the most Fleet Yaw induced variability of any projectile caliber & type. 6.8 mm had the least Fleet Yaw variations of any projectile caliber & type tested.

Projectile impact angle-of-Attack (AOA) variability is caused by bullet to bullet variations at impact and can substantially wound severity; this factor is more prevalent with certain calibers and projectile types. Testing demonstrated that 5.56 mm is highly susceptible to AOA variations, particularly when using FMJ projectiles such as M193 & M855. For example, with 5.56 mm FMJ, at higher AOA’s, for example 2-3 degrees, bullets had a shorter neck length (NL) and upset rapidly, thus providing adequate terminal effects; at low AOA, like 0-1 degree, the projectiles penetrated deeper than ideal prior to initial upset (ie. long NL) with significantly reduced terminal effects. Note that other calibers were less susceptible to AOA variations than 5.56 mm and OTM’s tend to have less AOA issues than FMJ. The 6.8 mm has proved to have the least AOA inconsistencies of any caliber tested to date.

Again, both projectile AOA and Fleet Yaw appear much more prevalent in 5.56 mm, particularly with FMJ loads, than with larger calibers and match loads. These are clearly multi-factorial problems of multiple etiology including: ammo and weapon design issues, ammo and weapon manufacturing variations, individual weapon wear issues, along with both mechanical function and internal ballistics variances.

[Armati]

Have you read this: http://www.dtic.mil/ndia/2008Intl/Roberts.pdf?

Awesome, thanks.

I have actually never seen this before.

One question, has anyone revisited the 6.5 Grendel with some of the new loadings?

It would seem to me that if we were going to transition to a single caliber the 6.5mm would be better in the DMR and SAW role while still offering everything and more that the 6.8mm offers.

[jmart]

What are you trying shoot at 400, 500, 600yds? Paper? Deer? Something else?

6.8 isn't terribly flat. You'll have to know your range pretty precisely to hit at those distances. Miscalculate range by 50 yards and you'll be off significantly.

What optic setup do you intend to run?