6.8 questions

[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]

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.

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.

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]

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.