6.8 questions

[JWR075]

I have got the urge to get a new upper in 6.8mm. Should I go DI or piston? I am looking at LWRC M6A2, LMT Defender, Noveske, Wilson, Barrett or PRI. Which do you prefer and why. Please no bashing as I am looking for real world experience with these brands. I am not trying to cause a b@*#h session, just want to get the best for my money.

[river rat richard]

I got a 16" barrel from ar precision.its 4150 with 5r rifeling and its melonited. go to the .68forums ,lotta good info.

[QuadBomb]

DI vs. Piston follows the same rules as 5.56. Piston is best for SBRs shorter than 12" or if you plan on running full auto a lot and with a suppressor.

+1 on 68forums. I have an AR15 Performance 6.8mm upper, it's high quality.

[Nevermiss]

68 forum is great. There are many great options. I wanted a SBR that I could run suppressed and I went with the Noveske 12.5" Crusader with switchblock and it came with a stripped Noveske lower!

Again, you will get great, honest info on the 6.8 forums and there are many great options and most similar uppers are also similarly priced.

[fdxpilot]

Another thumbs up AR Performance. They have everything from basic, military style to super customized hunting rigs. I have an 18" Stainless mid-length gas upper, with basic, military style handguards, and a YHM folding sight/gas block.

Harrison, the guy who runs AR Performance, will make you a gas-piston rig, but personally, I would stick with DI. Harrison is also one of the prime movers behind the higher performance SpecII/ 6.8x43 chamber, with 1 in 11-13 rifling. LMT and Barrett are still stuck with older specs.

[rsilvers]

As for DI vs Pistol - I feel that comes down to the gun and selecting based on that is a bit like selecting a car based on overhead cams vs pushrods. It is better to just get the most accurate, durable, and lightest rifle - whichever it is.

I tend to be suspicious of piston and consider DI the default and would tend to assume DI is better (lighter, simpler, suppresses better, smoother recoil, potentially more accurate) unless proven otherwise.

[PatEgan]

There are alot of good manufacturers, but stick with the following, no matter who makes it:
-SPEC II chamber
-1/11-1/13 twist rate
-3-5 groove.

I have a Ko-Tonics built upper that is a SPEC II chamber, 1/11 twist, with 4-groove rifling. Wicked accurate and a great combo of features. Log-in at 68forums and start reading the stickies...

Pat

[cigardad]

If going the DI route, it's hard to beat a Noveske. They already have the proper chambers and twist rate.

[rsilvers]

Why do you suggest 1:11 or slower twist rate? And 1:13 seems like a very poor choice.

The longest bullet one would want to use in 5.56mm with a 1:7 twist is a Sierra 80. It is 1.090 inches long. At 2600 fps, the Miller Twist stability at 0 degrees F and 2600 fps is 1.55. It seems like one should try to stay around 1.5 or more for Miller Twist, especially if one will use a sound suppressor and don't want a baffle strike.

So what are some longish 6.8 SPC bullets? Barnes and Nosler. Some people may want to shoot 130s, but they are really too long for the brass. So let us stick to 110. With 110 as the max....

For 6.8 spc, 2500 fps:

110 Accubond - 1.105 inches long
1:13 twist - 0.88
1:12 twist - 1.03
1:11 twist - 1.22
1:10 twist - 1.48
1:9 twist - 1.83

110 TSX 1.148 inches long
1:13 twist - 0.79
1:12 twist - 0.92
1:11 twist - 1.10
1:10 twist - 1.33
1:9 twist - 1.64

Something between 1:9 and 1:10 - closer to 1:9 - is the most like using 1:7 twist in a 5.56mm rifle. Expect lower velocity and lower stability with very short barrels, so I would probably go with 1:9 for an 8 inch barrel and 1:10 for a 16 inch barrel.

Another thing to consider - there is some evidence that faster twist increases terminal effects with certain bullets - which might be a reason to just jump to 1:9.

Now if you are trying to build a competition rifle, as opposed to a combat rifle, then 1:11 - 1:12 would seem ideal.

[500grains]

Originally Posted by rsilvers

Why do you suggest 1:11 or slower twist rate?

1:10 developed high pressures. With the early ammo, there were very serious high pressure problems. The slower twist addresses part of that problem. The spec II chamber addresses the other part.
[/quote]

[rsilvers]

Most of that were early barrels with bore and chamber dims at or below minimum size. Not having the SPC-II chamber is also significant but not by a huge amount. Twist is so small a factor it is not significant.

Remember, the military brought 5.56mm from 1:14 to 1:12 to 1:7 twist with very hot M193 NATO ammo with no significant difference in pressure.

No one has shown data which demonstrates that 1:10 twist 6.8 barrels would have significantly more pressure than 1:11 twist.

[BullittBoy]

I have a mid-length CMMG 1/11 4 groove that shoots MOA with Hornady 110 BTHP and sub MOA with 110 Accubonds.
They cost $600 retail, they are all milspec and come with chrome lined barrel and a bolt group for that price-hard to beat.

The newer SPCII 1/11-1/12 twist and 3-5 grooves barrel vs older 1/9.5-1/10 twist 6 groove barerels has proven to lower pressures and increase velocity by going this route.
If you want to ever hand load and reall get the potential out of the cartridge the 1/11 or 1/12 is the way to go. Pretty much all of the manufacturers except Ruger has the SPCII chamber, this is kind of like a .223 spec vs. 5.56 in an AR, why would you not want to have a chamber that can handle higher pressure ammo?

[rsilvers]

It has not been proven that 1:11 or 1:12 twist has measurably less pressure than 1:10 twist, nor would I expect it to. Why would you want to give up bullet stability under extreme conditions of cold weather and when using a sound suppressor and give up the terminal effects benefit of a faster twist with certain bullets when there is no significant pressure difference?

Every SAAMI 6.8 barrel is able to handle a 78,500 psi proof round so really a SAAMI chamber can handle enough pressure for any reasonable factory load - which should all be under 60,000 psi anyway.

[BullittBoy]

A. I live in Texas so "cold weather stability" doesn't really apply to me personally
B. I don't have a supressor so that does not apply to me either, BUT lots of folks do with 1/11 and 1/12 guns and they work just fine.

You need to go spend about 30 minutes on 68forums.com and check out all the "real world" people over there that use this cartridge every day-and have been using it for 5 years+.
I had a 1/10 gun in 2005 with SPEC I and then I "reamed" it to SPC II. My 1/11 gun now I get an extra 50-100 FPS with exact same loads and re-loads so it does help out a bit. The pressure signs will disappear in 1/11 guns that you will have gotten if you loaded the rounds hot in a 1/10.
My 1/11 is every bit as accurate if not more than my 1/10 was and I have the added feature of no pressure signs and extra velocity.
The "grooves" in the barrel are the pressure reducing aspect of this--most all the 1/10 guns are 6 groove, the 1/11-1/12 are 4 groove and 3 groove so less friction= less pressure on the brass.

[rsilvers]

Originally Posted by BullittBoy

The pressure signs will disappear in 1/11 guns that you will have gotten if you loaded the rounds hot in a 1/10.

That is true for some 1:10 barrels, but not because they are 1:10 - rather because of other variables such as bore cross-sectional area, minimal headspace, or minimal chamber dimensions. You can use a ball gauge to verify.

Originally Posted by BullittBoy

My 1/11 is every bit as accurate if not more than my 1/10 was and I have the added feature of no pressure signs and extra velocity.

Benchrest people believe slower twist is generally more accurate, so that does not surprise me. But we are talking combat rifles here - so versatility of function under more extremes of weather with a wider range of bullets would seem to be a reasonable way to go. In fact that is the decision that most people here made when they selected 1:7 twist barrels over 1:9 for 5.56mm. The bulk of the commercial AR industry went on a 10-year diversion into the medium-twist world (1:9) and the 6.8 world is on that path also.

Originally Posted by BullittBoy

The "grooves" in the barrel are the pressure reducing aspect of this--most all the 1/10 guns are 6 groove, the 1/11-1/12 are 4 groove and 3 groove so less friction= less pressure on the brass.

Pac-Nor makes 1:10 twist in 3 and 5 groove.

[DocGKR]

What is now called the SPCII is the ORIGINAL chamber that Cris Murray of AMU and Steve Holland at 5th SFG(A) used in all the 6.8 mm test guns circa 2002--as noted, the whole chamber issue is kind of like .223 (6.8 mm SAAMI) vs. 5.56 mm (Murray/SPCII). These two progenitors of 6.8 mm both recommend the use of 1/11 or 1/12, 3 or 5 groove barrels for use in typical civilian rifles with 16" barrels. When using SBR's, a 1/10 or faster twist helps compensate for the rotational velocity loss that can reduce terminal effectiveness of some ammunition when used with shorter barrels. Personally, I use SPCII chambers, along with both 1/10 and 1/11 barrels and don't loose sleep over this...

[PatEgan]

Originally Posted by rsilvers

It has not been proven that 1:11 or 1:12 twist has measurably less pressure than 1:10 twist, nor would I expect it to. Why would you want to give up bullet stability under extreme conditions of cold weather and when using a sound suppressor and give up the terminal effects benefit of a faster twist with certain bullets when there is no significant pressure difference?

Every SAAMI 6.8 barrel is able to handle a 78,500 psi proof round so really a SAAMI chamber can handle enough pressure for any reasonable factory load - which should all be under 60,000 psi anyway.

I would suggest posting this over on the 68forums for the broadest and deepest range of informed answers. As another poster mentioned, they are the longest running users of this caliber. Their findings are based on the kind of experience that comes from failure, success, and what led to each. I think you might be using a little too much theory and inappropriate comparison in your analysis.
Pat

[rsilvers]

I did look over there, but they are focused on hunting with high-pressure (60,000+ psi?) hand loads and for longer barrels. I am more into the combat rifle thing. Not really the same goals.

[PatEgan]

????
Not really, most members there are running rigs in the 16" range, set up for taking two-legged prey.
Pat

[rsilvers]

You are right, most are.

But really, when I see higher velocity on certain barrels, it makes we want to measure the chambers to know their dimensions. That is the only way I can hope to know the effect of the other barrel parameters.