Average muzzle velocity for this load from the 8.5-inch barrel measured a respectable 2450 fps. In doing so, this load generates 1199 foot-pounds of energy at the muzzle. Firing the same load from a 16-inch barrel produces 2900 fps and 1680 foot-pounds.
Taking barrel length to 24 inches increased velocity to 3050 fps and energy to 1858 foot-pounds. Testing in extreme temperatures showed velocity to average 2275 fps at -29 degrees F and 2475 fps at 125 degrees F from the 8.5-inch barrel. When fired from a 24-inch test barrel, ATK recorded a group measuring 1.56 inches at 200 yards.
The problem is with all 6.8 ammo, not just the Federal UCIW Six8 ammo. 6.8 magazines generally aren’t very reliable due to the larger case diameter, and they are only 20-25 rounds. The LWRC lower accepts a proprietary magazine, designed by Magpul, that features an optimized double stack configuration, but it won’t fit in a standard AR15 magazine well.
The mag issue is cited in the article, and is related to full-auto fire. Per LWRC’s posts on their own forum 1 magazine induced failure in 800 rds full-auto wasn’t good enough. FWIW I have ~5000 rds through a 6.8 now, and have yet to have a failure (using PRI mags primarily). I don’t do many mag dumps or peel drills with this round as my rifles are set up for hunting. This cartridge’s appeal to me transcends the quick change AR15 upper, so I’ll probably switch to the new platform.
I corrected you the last time, and you keep on misrepresenting my position. I have never said that an SSA tactical load was dangerous. Please quote me say that from anywhere on the internet.
What I did say was that:
5.56mm is 0.332 inches in diameter on the case head, which is 0.08657 square inches. 62,000 pounds per square inch is 5313 pounds into the bolt.
6.8 has a case head of 0.358, which is an area of 0.10066 square inches. In order for this also to have 5313 pounds into the bolt, you need to limit it to 52,785 psi.
To be clear - 62,000 psi for 5.56mm is the same as 52,785 psi for 6.8.
Moreover, the 6.8 bolt is weaker than a 5.56mm bolt.
If the 6.8 spec was raised to 58,000 psi, it would be like 5.56mm at 68,000 psi.
If the 6.8 spec was raised to 60,000 psi, it would be like 5.56mm at 70,500 psi.
So my point was, one cannot use 5.56mm pressure and 6.8 SPC pressure interchangeably, and so that it did not make good sense to load 6.8 over SAAMI max pressure on purpose.
Limiting the scope of the comparison to a AR-15 BCG as a means of writing off the caliber is a little short sighted, don’t you think? If dedicated 6.8 lowers are on the horizon, then dedicated 6.8 uppers with stronger BCG designs are a logical evolution should the 6.8 take off any further.
It seems once a new caliber or two comes out we start ditching all of the calibers that had our praise before their arrival…Simple question: Is 6.8 a worthy caliber for a designated marksman setup?
Can’t answer anythoing for the 6.8 SPC, but have heard through the grape vine of various problems that have in one instance or another , realized its fate as a civilian cartridge and not a military designated cartridge. I also can’t say anythoing about the 6.5 Grendel, the .264 LBC, or 6.5 Model1 as I do not own and more than likely will never own one. As far as anything that I have read about the two, I would opt for the 6.5 versions. Mostly due to a better selection of bullets with a bit higher BC. I own several antique military and modern bolt guns in 6.5 x 55 mm, as well as an AR15 chambered in 6.5 PCC (Patriot Combat Cartridge). Basically the 6.5 PCC is a .223 case cut to 1.645", fireformed to the chamber (different shoulder angle than .223), and resized to hold roughly 33 gr of H2O instead of 30 gr H2O that the parent .223 case can hold. It is not a screamer because the 6.5 bullets take up a lot of case space and the magazines restrict it to 2.300" (if you use the HK416 or PRi mags) or 2.260" if you use standard Mil-Spec mags. It is highly accurate and plenty good for most big (deer/ pig sized) game animals out to 300 yards using 120 gr bullets @ 2800 fps in a 24" barrel. Good luck on your choice, it is a lot of fun when you have to choose.
I like the 6.8 and have killed a bunch of animals with it—I consider it the perfect AR hunting cartridge. However, I think the Brits nailed the designated marksman rifle with the L129a1. A 16" .308 makes 600yd shots seem easy—a 5.56 or 6.8 is a bit harder, especially in the wind.
With that said, factory Hornady 110gr BTHP ammo has essentially the same external ballistics as MK262 but delivers ~50% more energy on target. Also I have fired MK262 and factory 110 BTHP to 1000yds (16" 6.8 & 17" 5.56) side by side a couple times and my hits were more consistent and noticeable with the 6.8. As for the additional energy, I’m not sure it matters inside 200yds, but beyond it certainly could make a difference. Drawing from my experience I think the 6.8 is a probably a better option than .223 in a DM type role, but also think it is easily outclassed by .308.
