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]

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.
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