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6.8 barrel twist rates revisted

22764 Views 35 Replies 20 Participants Last post by  marinesg1012
6.8 barrel twist rates revisited

Twist discussion for the 6.8 SPC has surely been covered, but didn't find much via the search in threads with "twist" in the title. Perhaps detailed info is in threads with different titles, but thought a dedicated thread wouldn't be such a bad idea...

It seems that 1:11 is the preferred twist rate, but many barrels are 1:10. So are the 1:10 barrels no good, or are the 1:11 barrels simply better? How much difference is there?

What can be expected with 1:10 barrels: less velocity, less long-range accuracy, less life, bullet failures? I understand that pressure is more with the faster twist, but I have to assume that it's not unsafe. So how does the extra pressure affect performance?

Seems that 1:10 uppers are available, but 1:11 uppers are more difficult to find unless the buyer is willing to order and wait.

I assume that there are numerous 1:10 owners on the forum. What's your take?

Just wondering if 1:10 should be completely avoided, and to what extent the 1:11 is better.

Any feedback would be appreciated.

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If you are in the market for a 6.8, you really need to purchase or order SPC II chamber or DMR chamber and the 1:11 or 1:12 twist. The 1:10 barrel is largely going extinct in the 6.8 market.
Kind of like buying the old tube television when everyone else is buying LCD's or Plasmas and going HD.

Keep in mind that over the past several months to a year... the vast majority of the 6.8 firearms manufacturers have moved to 1:11 or 1:12 barrels and the new chambers, or are in the process of moving to the new twists.

Ammunition can be safely loaded to higher performance levels for barrels with the improved chambers and slower twists. There can be a 200 feet per second difference between an obsolete SAAMI chamber and a 1:10 barrel compared to SPC II, or DMR chambered rifle with 1:11 or 1:12 twist at "safe" pressure levels. In the rifle world that would be the difference between a standard caliber and a magnum. That's a huge difference out of the same cartridge just by going to better barrel specs.

Armalite - new 6.8 barrels on order with SPC II and 1:11 4-groove
Bushmaster - new barrels coming, SPC II and 1:11
DPMS - new barrels coming SPC II and 1:11
CMMG - M10 6.8 barrels in stock now..SPC II and 1:11
Patriot Ordinance Factory - SPC II and 1:11 poly
Stag - SPC II and 1:11
White Oak Armament - new Shilen blanks 1:11 4-groove, SPC II
Noveske - Mod 0 (similar to SPC II) and 1:12 poly
Bison Armory - SS SPC II 1:11 4-groove
AR15Performance and Titan Armory - DMR and 1:11
ADDAX Tactical - SPC II and 1:11 .

If you're new to the forum or the 6.8, you really should read the 6.8 barrel performance test report that was conducted last summer. I think Adam posted it above, and I'll put it in my post as well.

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"2) the rifling design. Believe it or not, the design of the bore and ratio of lands to grooves seems even more important than rate of twist. Ergo, if you had a 1 :10 twist Rock 5R barrel with 75:25 lands to grooves ratio, I submit you would see less pressure with that barrel than one having 1:11 twist, 6 grooves, slightly irregular chrome lining and a not-so-perfect bore." HTR.

HTR's post is technically better than what I posted. There aren't many 6-groove 1:11's out there any more so worrying about that isn't that big of a deal. Even then, the Denny's GTS 1:11 6-groove barrel ran with much lower pressure than the DPMS SAAMI 1:10 so you're still seeing a major improvement over the obsolete SAAMI chambered 1-10 barrel tested.

A little pressure difference and velocity difference between manufacturers is one thing, and realistically expected depending on the rifling type, barrel material, quality, lining, etc. However there are huge pressure differences between the obsolete SAAMI specs and the newer SPC II, DMR and slower twist, or more efficient rifling designs on the market today. That's the point you need to focus on, not the small differences among the manufacturers who have moved to the newer specs.

Keep in mind that most of the AR manufacturers are going to SPC II and 4-groove 1:11 barrels which should definitely perform better than their previous 6-groove 1:10 offerings, especially with the SPC II chambers. You always have your custom or premium barrels with different types of rifling and twists that may or may not offer a performance advantage. Having the major production companies like Armalite, Bushmaster, DPMS, Stag, CMMG and others going to SPC II and 4-groove 1:11 is a big step in the 6.8's evolution.

It's funny how we're discussing relative details now. There are so many good choices out there for 6.8 shooters and more are coming. Just a year or two ago and we were discussing the need for major changes for better performance. Much has changed in this short time frame. Largely the entire industry has moved to better specs and now we're discussing refinements instead of revolutionary changes.
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... Good luck with that 1:8 twist barrel.
Colorado is above sea level. A bullet stable at high altitude may not be stable at standard air density. Likewise, a bullet in Texas at warm temps won't necessarily be stable at freezing temps.
Are you related to StanC ? Just askin'. I see a family resemblence.
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