6.8 SPC Barrel Twist Rate?

[CoryCop25]

What is the proper twist rate for a 6.8 SPC barrel? I have seen 1/10, 1/11 and 1/12. I would most likely use 110 and 115 gr ammunition.

[Abraxas]

I have been told 1/10. I don't know if that is correct, but it is what Noveske has for his 6.8

[Thomas M-4]

I have been told 1/10. I don't know if that is correct, but it is what Noveske has for his 6.8

http://noveskerifleworks.com/cgi-bin...since=&status=
Not any more novoske uses 1/12 and a spec 2 chamber
The 1/10 twist rate is standard for the 270 win when the 6.8 came out that is what they used. The 1/11 and 1/12 twist rates work better for the spec 2 chambers.

[Thomas M-4]

The longer free bore of the spec 2 chamber and the slower twist rate will allow you to safely use SSA combat loads which are loaded to higher pressure.

For shooters who'd like even more performance, and who have the proper rifle, SSA offers a velocity step-up. Its 85-grain TSX Combat Load features the same Barnes bullet, but driven to an impressive 3,000 fps. Yes, that's 3,000 fps from a 16-inch AR.

Terminal performance of this load promises to be even better than the standard load.

[Robb Jensen]

IIRC Dr. Gary Roberts says the 1:11 with SPC II is the ideal combo. That's the twist and chamber my White Oak 6.8 barrel is.

[SHIVAN]

Noveske used to use the 1:10 SAAMI spec chamber, but got enough requests to switch over to the 1:12 0.100" freebore, more commonly known as the SPECII chamber.

White Oak doesn't specify SAAMI, SPECII or otherwise last time I conversed with him on email, but he does 1:11 with 0.100" freebore, so it is in fact SPECII.

[BWilson]

We think 1-11 4 groove w/SPC II chamber will be industry standard in the near future, maybe called 6.8x43 instead of 6.8SPC

[DocGKR]

The original Murray/SPCII chamber rather than the overly tight Remington SAAMI chamber is the critical factor in proper 6.8 mm performance--much more so than barrel twist, just like it is critical to ensure a true 5.56 mm chamber instead of a SAAMI .223 one.

While it is clear that 6.8 mm rifles with barrels around 16" firing typical 115 gr and under projectiles work very well with 1/11-1/12 twist barrels, in some cases slower barrel twists can create terminal performance problems with certain projectiles launched from very short barrels. For very short barrel weapons, a faster twist rate may be necessary to ensure an appropriate rotational velocity to maximize the terminal performance of some types of projectiles...

As an example of rotational velocity issues, lets look at a hypothetical 5.56 mm JSP projectile launched from a 20" 1/10 twist barrel with a MV of 2900 f/s; the projectile hits a target at 300 yds with an impact velocity of 2100 fps and expands normally. Yet when that same projectile is launched from say an 8" 1/10 twist barrel with a MV of 2100 fps and hits a target 3 yds away, the projectile fails to expand. Why? The impact velocities are the same, so why did the projectile launched from the short barrel fail to upset? New research indicates that a major variance is the difference in rotational velocity. While the impact velocities for the two shots are the same, the shot from the longer barrel has a much higher retained rotational velocity at 300 yds--in this case say 35% higher than even the initial rotational velocity of the SBR, since rotational velocity decays at a much slower rate than linear velocity.

[M16MANIAC]

The original Murray/SPCII chamber rather than the overly tight Remington SAAMI chamber is the critical factor in proper 6.8 mm performance--much more so than barrel twist, just like it is critical to ensure a true 5.56 mm chamber instead of a SAAMI .223 one.

While it is clear that 6.8 mm rifles with barrels around 16" firing typical 115 gr and under projectiles work very well with 1/11-1/12 twist barrels, in some cases slower barrel twists can create terminal performance problems with certain projectiles launched from very short barrels. For very short barrel weapons, a faster twist rate may be necessary to ensure an appropriate rotational velocity to maximize the terminal performance of some types of projectiles...

As an example of rotational velocity issues, lets look at a hypothetical 5.56 mm JSP projectile launched from a 20" 1/10 twist barrel with a MV of 2900 f/s; the projectile hits a target at 300 yds with an impact velocity of 2100 fps and expands normally. Yet when that same projectile is launched from say an 8" 1/10 twist barrel with a MV of 2100 fps and hits a target 3 yds away, the projectile fails to expand. Why? The impact velocities are the same, so why did the projectile launched from the short barrel fail to upset? New research indicates that a major variance is the difference in rotational velocity. While the impact velocities for the two shots are the same, the shot from the longer barrel has a much higher retained rotational velocity at 300 yds--in this case say 35% higher than even the initial rotational velocity of the SBR, since rotational velocity decays at a much slower rate than linear velocity.

Thanks for sharing this info, I have never thought about RV in terms of projectile performance.

[BWilson]

The original Murray/SPCII chamber rather than the overly tight Remington SAAMI chamber is the critical factor in proper 6.8 mm performance--much more so than barrel twist, just like it is critical to ensure a true 5.56 mm chamber instead of a SAAMI .223 one.

While it is clear that 6.8 mm rifles with barrels around 16" firing typical 115 gr and under projectiles work very well with 1/11-1/12 twist barrels, in some cases slower barrel twists can create terminal performance problems with certain projectiles launched from very short barrels. For very short barrel weapons, a faster twist rate may be necessary to ensure an appropriate rotational velocity to maximize the terminal performance of some types of projectiles...

As an example of rotational velocity issues, lets look at a hypothetical 5.56 mm JSP projectile launched from a 20" 1/10 twist barrel with a MV of 2900 f/s; the projectile hits a target at 300 yds with an impact velocity of 2100 fps and expands normally. Yet when that same projectile is launched from say an 8" 1/10 twist barrel with a MV of 2100 fps and hits a target 3 yds away, the projectile fails to expand. Why? The impact velocities are the same, so why did the projectile launched from the short barrel fail to upset? New research indicates that a major variance is the difference in rotational velocity. While the impact velocities for the two shots are the same, the shot from the longer barrel has a much higher retained rotational velocity at 300 yds--in this case say 35% higher than even the initial rotational velocity of the SBR, since rotational velocity decays at a much slower rate than linear velocity.

Some very good points from an expert on the subject of which I admit I have very little personal knowledge. I agree rate of twist isn't a major factor in reducing chamber pressure, however #, size and shape of the grooves does seem to have a noticable effect.

My main use of the 6.8 is for hunting, mostly hogs, deer and predators. Here I do have some knowledge and a lot of experience. We (guests, clients and myself) have shot 40+ hogs and 20+ deer YTD with the 6.8 as well as done a fair amount of bullet performance testing on large deceased hogs. All has been with 1-11 twist barrels ranging from 11" to 18" with most being 4 groove. Muzzzle velocities for 110gr bullets has been 2325-2600fps depending on load and barrel length. The following bullets have reliably performed as to be expected for their respective design.

Barnes TSX and TTSX
Nosler Accubond
Sierra Pro Hunter

I have even used the 130gr Nosler Accubond in my 11" SBR (MV 2150fps) on several hogs will good success.

DPMS has now standardized on a SPC II chamber and 1-11 twist and it is my understanding that Bushmaster is in the process of doing so too.

GKR tks for the this and the previous info you have shared with me.