Which round for big game AR-15?

[Todd.K]

That narrative is just plain wrong. Remington wasn't involved in development for one. By mistake or on purpose the AMU sent Remington the the "match" or "wylde" chamber print rather than the "NATO" version. Remington just took what they got to SAAMI.

This accounted for a very small amount of the difference between pressure tested loads and the wildcat velocity claims. The majority was simply handloading to higher pressure than intended. This has been proven in ballistics labs.

[C. Stevens]

The SAAMI chamber has a wrong number somewhere, in the leade, I believe. It all came down to the way Remington handled the project. At first, they supported it, and helped the individuals R&D the cartridge. However, a lot of it was to copy and paste from the 270 Win, 'cause 270 is 6.8. Some folks realized that things are different in an AR platform, hence the many "experiments." Then, it seemed that one day Remington stopped all work, took what they had to SAAMI, then froze the cartridge and washed its corporate hands. What started out as 6.8 Remington become an orphan, and morphed into 6.8 SPC, with more development work done by private individuals, without corporate backing. The chamber is different, the pressures are higher (and not blowing up the guns), the SAAMI standard is restrictive and obsolete, yet no one can change it -because no big money company is willing to touch it. Anything else folks are doing is a "wildcat" -no matter how safe or successful. And a decade too late. So, the 6.8 SPC is an "also run", an evolutionary dead end.
Do you want to talk about "pressure breaking bolts?" Let's talk 6.5 Grendel breaking bolts made for the 7.62x39 cartridge -because manufacturers are cheap or don't know there is a different bolt face for the Grendel. Do I blame the cartridge? No!
The Grendel offers the best performance at longer distances, at 600, 800, even 1,000 yards. Better than any 6.8 or 5.56. There is a 70+ pages thread in GD on TOS, for the non-believers.
For humane hunting at ethical distances, with an AR15 platform, both sixes will do great. I stand by my suggestion that bullet selection and ammo availability should be the deciding factor.

Remington abandoned the 30 Remington AR also... They always start something good and can't follow through. They are marketing failures.

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[constructor]

If it takes 3000-5000psi to increase the velocity of a bullet 100fps and you reduce the 3000-3500psi engraving force by 40% and increase the freebore by .050" how much powder can you add to a cartridge to bring the max PSI back up to the same max PSI before the changes? Now that there is more powder in the case does it change the area under the pressure curve? What does changing the area under the curve do? Does that additional grain of powder in the case increase the velocity?
Can one manipulate the bore/rifling to increase or decrease velocity? Why are some barrels of the same length faster than other brands of the same length?

[constructor]

That narrative is just plain wrong. Remington wasn't involved in development for one. By mistake or on purpose the AMU sent Remington the the "match" or "wylde" chamber print rather than the "NATO" version. Remington just took what they got to SAAMI.

This accounted for a very small amount of the difference between pressure tested loads and the wildcat velocity claims. The majority was simply handloading to higher pressure than intended. This has been proven in ballistics labs.

The Wylde freebore is actually longer than the NATO freebore-The NATO has a .566 freebore length and the Wylde has a .0619 freebore length.

I have only seen one drawing from the AMU but as the story goes both 6.8 drawings from the AMU had the same case and freebore diameters, the only difference was the length of the freebore. The SAAMI has a .050" freebore, the SPCII has a .100" freebore. There were several mistakes made along the line including a transposed number by the reamer grinder which left a sharp angle at the beginning of the freebore. The copper clad Extreme bullets deformed upon firing pushing the lead into the small cone area then it was shaved off by the sharp edge leaving a copper ring in the cone area. As more rounds were fired the copper rings compressed and forged themselves together creating a choke point.

The ballistic labs only tested the barrel blanks they had in stock, all had the same rifling and none had all of the changes combined in the same barrel. Guys that shoot benchrest have been using tricks for the lack of a better word for years. Guys that shoot ARs usually do not experiment enough to figure it out. In the beginning all of the AMUs barrels were made from Douglas blanks, that could explain why the AMU was getting such good performance from the round. It wasn't until Remington got involved and made some production grade 10 twist 6 groove barrels and large primer ammo in a .418" diameter case that all of the velocities dropped. Remington decided to abandon the project and that was when Barrett and SSA got involved. Kind of hard to believe SSA and Barrett did not test their products together before showing up at a base for a demonstration. Lots of huge mistakes from everyone involved.

[MisterHelix]

If it takes 3000-5000psi to increase the velocity of a bullet 100fps and you reduce the 3000-3500psi engraving force by 40% and increase the freebore by .050" how much powder can you add to a cartridge to bring the max PSI back up to the same max PSI before the changes? Now that there is more powder in the case does it change the area under the pressure curve? What does changing the area under the curve do? Does that additional grain of powder in the case increase the velocity?
Can one manipulate the bore/rifling to increase or decrease velocity? Why are some barrels of the same length faster than other brands of the same length?

I do not know the answers to all those questions, but you've obviously done your homework.

Whatever it is you're doing, keep doing it.

My AR Performance 6.8x43 16" 5R barrel clocks velocities faster than manufacturers specs:
Hornady 110 hpbt, factory listed velocity from a 24" test barrel is 2,570. My 16" ARP chronos at 2,694

Likewise, the starting load from Nosler for the 100 grain Accubond, 29 grains of Benchmark, is listed as 2,569 from a 20" Pac-Nor. My 16" ARP throws them at 2,600 fps.

So, whatever chamber/twist/spec calculations and modifications you've done have objectively improved the performance of the 6.8spc.

So, thanks and stuff.

[tom12.7]

We could look back into a history of what H&H and others have done in the past and make some comparisons and look into some results.

[Todd.K]

Whatifing a bunch of little "tricks" is not what SAAMI is about. It's about a safe standard for mass production. It's why a pressure test barrel is made to minimum bore and chamber dimensions.

The 6.8 is what it is. I think it's pretty good, especially when loaded with a bit lighter bonded or all copper bullets.

I also know the constant complaints have kept a whole lot of people from finding out just how good the 6.8 actually is. None of this will make the OP want to choose the 6.8.....

[constructor]

Whatifing a bunch of little "tricks" is not what SAAMI is about. It's about a safe standard for mass production. It's why a pressure test barrel is made to minimum bore and chamber dimensions.

The 6.8 is what it is. I think it's pretty good, especially when loaded with a bit lighter bonded or all copper bullets.

I also know the constant complaints have kept a whole lot of people from finding out just how good the 6.8 actually is. None of this will make the OP want to choose the 6.8.....

Yeah, I'm good with that and I'll take the bag o tricks behind door #1. SAAMI can keep their Remington test barrels.
SSA and I were trying to prove to the military that there was more performance in the 6.8 when all things were designed correctly instead of using whatever left over 270 Win blank with 10 twist and 6 groove enfield rifling they used. I made the 2nd proof barrel for SSA summer 2008. If you were trying to sell the military on an idea would you approach them with the model that was 100fps faster or the one that was 100fps slower(same peak pressure)? Mattis was all for the 6.8 but the budget was tight and he decided he didn't want to work for the gay O. Stuff happens, the 6.8 didn't make it as a US military cartridge but it's one hell of a deer and hog round and the "hunting" crowd is a lot bigger than the "long range" crowd. It played a part in making the MSR more popular for hunting and the pyramid will continue to grow.

[TacMedic556]

All I can say is, here in Montana several of us hunt with the 6.8 SPC II (16" 1:11 twist) rifles and have seen absolute and tremendous success. The Mule Deer buck I took last fall was a large bodied buck, struck at 350 yards and nose dived into the dirt in front of his face. When we chrono'd rounds out of the 6.8s we were shooting 50-100 fps slower than full size .270 pre-64 Winchesters that were shooting 140 grain Nosler Accubonds. The 110 grain 6.8 is a great round for deer and antelope out to the 300-400 yards. I would have no issue, with the superb accuracy and energy deposit, taking a buck at that range. According to ballistic table, my 6.8 with a 110 Grain should be hitting at 760 ft. lbs. at 400 yards and 619 ft. lbs at 500 yards. That is comparable to the hottest 10mm loads out of a Glock at the muzzle. I hear some of the same guys who argue against the 6.8 SPC for a deer (180-200 lbs animal) at those distances, say they would carry a 10mm for Grizzly (400-600 lbs animal). Just an interesting point I think. I have seen deer drop with a 9mm. I have no issue or concern of slamming a deer at distance with a 6.8 and can tell you from personal experience that the accuracy is pristine. Ballistic coefficients and paper punching are for exactly that - punching paper. If I am shooting game with an AR the 6.8 is the minimum I would take for that task.

The more the 6.8 can gain support and grow, the more bullets and loads we will see come available. What needs to happen is a hot load of 115 or 120 grain bonded bullets that are specifically designed for the 6.8 game hunting. We spent considerable time debating which of the non 5.56 cartridges we wanted to invest in when choosing the 6.8. We weighed a lot of information, read a lot of material and landed where we did.

The 6.8 SPC II is an exemplary choice where one can have a round that is in the middle of the 5.56 and .308, while maintaining the same chassis as the standard AR-15 platform. Get a 6.8 and you will never regret it.

In 2 weeks I will post a picture of this years buck - taken with a 6.8 (fingers crossed!).

[Nightvisionary]

OP I went down this road over the last 7 years or so. I researched the 6.8 and 6.5 heavily but did not see enough of an increase in ballistics to justify to added costs of magazines, bolts, and brass. The 6.5 Grendel didn't attain the velocities with mid length barrels that I felt would give 1900-2000 feet per second needed for reliable downrange bullet expansion . I ended up building and reloading for 6x45 and 300 Blackout and sold both. I eventually came full circle and purchased a DPMS GII Hunter in 308. With a 20 inch barrel it is only one inch longer than my 5.56 AR carbines and at 7.25 lbs lighter than most of them.