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CoryCop25
11-27-09, 18:11
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
11-27-09, 19:20
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
11-27-09, 19:37
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/imcart/display.cgi?item_id=b-16-68&cat=18&page=1&search=&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
11-27-09, 19:51
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
11-27-09, 20:01
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
11-27-09, 20:33
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
12-04-09, 21:46
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
12-05-09, 01:14
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
12-05-09, 08:51
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
12-05-09, 09:21
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.

BLACK LION
12-08-09, 13:50
G-M4, my WOA is the 1-11 twist 6 groove stainless barrel from a wilson blank...
In 09 they changed to 1-11 twist 4 groove stainless barrels from shilen blanks...

Definately getting better.....

BLACK LION
12-08-09, 14:12
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.

Could this be why companies like LWRC are "sticking with" the 1/10 twist???

bloodyspartan
12-17-09, 10:00
Bushmaster is now selling barrels in the 1:11 SPC II format.

I have one. Not to shabby.

Marcus L.
12-24-09, 09:51
....so, what would be the ideal twist rate for a 16" SPC II rifle? How about a 12" barrel?

BWilson
12-24-09, 10:00
....so, what would be the ideal twist rate for a 16" SPC II rifle? How about a 12" barrel?

I only have experience with 1-11 and they work great for me

bloodyspartan
12-24-09, 21:37
....so, what would be the ideal twist rate for a 16" SPC II rifle? How about a 12" barrel?

Go to the 68 forum they feel I think that 1:12 3 groove or 5 groove. It depends on the width of the grooves.

They have done some good research also check out the arperformance website.

It might add some food for thought.

I am a pretty sure the 1:11 4 groove is pretty good also. They all seem to work and shoot it's just if you expect to reload or use high performance ammo the changes give you pressure room.

BWilson
12-25-09, 10:07
Go to the 68 forum they feel I think that 1:12 3 groove or 5 groove. It depends on the width of the grooves.

They have done some good research also check out the arperformance website.

It might add some food for thought.

I am a pretty sure the 1:11 4 groove is pretty good also. They all seem to work and shoot it's just if you expect to reload or use high performance ammo the changes give you pressure room.

I spend a lot of time on the 68 forum

Based on my research I'm betting that 1-11 4 groove will become the new "standard" for the 6.8x43 or whatever the new higher performance cartridge will be called. Not to say that 1-12, 3 groove, polygon, 5R, etc all don't have merit which thay all do. But keep in mind what Dr Robers posted here on this thread about the rotational velocity too.

Our stainless 1-11 4 groove bbls have so far proven to shoot extremely well and in some cases just as well as some big name bbls costing twice as much.

ccoker
12-25-09, 11:49
Our stainless 1-11 4 groove bbls have so far proven to shoot extremely well and in some cases just as well as some big name bbls costing twice as much.

I can vouch for that!
Personally speaking and from a number of target pics from Mr Wilson and of course, a pile of dead hogs and deer

AKsarben
12-28-09, 17:37
I had kind of steered away from Wilson Combat because I thought their barrels were the 1:10 twist. I have read that the 3 grove barrels are really sought out by bench rest shooters for their accuracy. I think 4 is good, 6 might work if the lands to grove ratio is in sync with what the 6.8 SPC II likes.

I agree, there needs to be a name for the 6.8 SPC II chambered rifles/carbines, and it would help define the "standard". Maybe, 6.8 XT for the XTra leade in the throad and the XTra fast bullet.? ;)

BTW, I think the 6.8 is an excellent candidate for the piston systems, now that they have pretty much dealt with carrier tilt.

Marcus L.
12-31-09, 13:05
For those of you with Robinson Arms XCR rifles, they are now using the 1/11 twist rate with SPCII chamber and 4-groves in their 6.8 barrels. :)

BLACK LION
12-31-09, 15:32
For those of you with Robinson Arms XCR rifles, they are now using the 1/11 twist rate with SPCII chamber and 4-groves in their 6.8 barrels. :)

This will be my next purchase since that was what I was waiting on from them...

The 11 twist 4 groove IMHO will be the umbrella specs for the 6.8x43 since it performs the best with projos from 85gr to 130gr... My forthcoming Xtreme is a nitrided 1-11 /5R with the DMR-C chamber... Should prve to be the all around workhorse...

1-12/1-13 with 2 and 3 grooves are what I consider "super match" specs for those that want the tightest accuracy possible and best performance from "hotter" loads...


I use the tubbs ff on all my uppers for the break in which makes real shooters out of these barrels and eases cleaning like you wouldnt belive...

Marcus L.
01-01-10, 09:30
This will be my next purchase since that was what I was waiting on from them...

The 11 twist 4 groove IMHO will be the umbrella specs for the 6.8x43 since it performs the best with projos from 85gr to 130gr... My forthcoming Xtreme is a nitrided 1-11 /5R with the DMR-C chamber... Should prve to be the all around workhorse...

1-12/1-13 with 2 and 3 grooves are what I consider "super match" specs for those that want the tightest accuracy possible and best performance from "hotter" loads...


I use the tubbs ff on all my uppers for the break in which makes real shooters out of these barrels and eases cleaning like you wouldnt belive...

Their website still lists their conversion as being a 1/10 twist, but they said that they haven't used the 1/10 for almost a year. Still, when I order mine I plan to do it over the phone to remind them that I want the new configuration.

AKsarben
01-01-10, 11:10
I just wonder how much hotter you can load a SPC II chamber compared to the DMR-C chamber. The latter was sort of a middle ground between the 0.100" leade and the 0. 050" lead of the original SPC I chamber. I believe it was to give better accuracy and with less pressure.

Cohibra45
01-01-10, 12:07
I just wonder how much hotter you can load a SPC II chamber compared to the DMR-C chamber. The latter was sort of a middle ground between the 0.100" leade and the 0. 050" lead of the original SPC I chamber. I believe it was to give better accuracy and with less pressure.

If you go over to the 68forums.com site and do a lot of reading you will find that those guys have about maxed out the potential of the 6.8. Constructor and TimW have been working up loads for a few years with different barrel/chamber/twist configurations.

Basically, your not going to see a few hundred fps more from handloading. It will depend on which barrel/chamber/twist/# and type grooves to achieve the maximum 'nth'!!! The difference in fps between 1/11-1/12 and 4 or three groove is very small. Really, what do you want???

There is no magic bullet. It's just as each and every rifle using the same combination will shoot a little different.

To answer your question or thoughts about can you shoot hotter loads out of a SPC II or DMR, both can handle the warmest loads out there. The SPC II is the more commercial chamber and DMR is Constructor's iteration. DMR is more a match chamber if you are shooting bench and handloading to suit your needs.

I believe the leade angle difference between the two chambers is more important in getting the 'max' out of the 6.8 than the length of leade.......

As DocGKR says, just make sure that the barrel you get is at least SPC II and 1/10 twist. Yes, 1/11-12 twist with less lands and grooves will get you more fps, but it is the chamber that will keep you out of trouble more often than not.;)

thmpr
01-01-10, 14:05
Why is Barrett still using 1"10 Twist rate if others are steering in the other direction?

DocGKR
01-01-10, 15:31
Same reason LWRCI has stuck with 1/10--because 1/10 works just fine when shooting the factory loads that all military and LE folks use and the faster twist can be beneficial for shorter barrel weapons.

AKsarben
03-26-10, 23:08
Talked at lengths this evening to Brian at DEZ arms in Wisconsin. They use a 1 in 10.5 (aka 10 1/2) twist 6 groove barrel with 68 X 43 SPC II modified angle chambers. The reason they went with 1 in 10.5 is that they felt it gave the best stability for 100gr and up, especially the 110 gr 115 and 130 grain bullets. It is what the military was wanting, a twist rate for 110 grain and better bullets. As Brian said, is one keeps going down in grain weight 85 gr 80 gr, pretty soon you might be in the same category as a 5.56 NATO round shooting the 75 grain bullet.

Monday he is going to send me some pics from folks that have used the 6.8 SPC DEZ Arms barrels that have shot one hole groups. They say they like to pay particular attention to details in the complete build of these barrels, and just like the 1 in 10.5 twist with the SPC II chamber. Their barrels handle any of the hotter loads, and have a 0.086" gas port on their carbine length systems. The last part after the gas port is drilled is that they use a special finishing broach that takes out the burr from the drilling of the gas port. The first round will take it off, but many customers do not like the idea of that burr in there from the making of the barrel. And, for their price on their barrels, and that they hand fit and make the barrel extension, and cut for M4 as well, assure a very well made barrel.

One of the things I really like about them is that they are not chrome lined. I consider that a plus. Shooting in a barrel that is the hardness level they have their barrels gives the lands and grooves a chance to really settle in.
www.dezarms.com is their web site.

BWilson
03-26-10, 23:23
The new 95gr Barnes TTSX is the DO ALL bullet for the 6.8 and seems to shoot very good in 1-11 twist bbls.

This bullet loaded to 2.285" over 29.5gr of R10X will do anything the 6.8 is capable of....................

t_ridl
03-27-10, 01:15
The new 95gr Barnes TTSX is the DO ALL bullet for the 6.8 and seems to shoot very good in 1-11 twist bbls.

This bullet loaded to 2.285" over 29.5gr of R10X will do anything the 6.8 is capable of....................

Thats awesome....I cant wait to try some. Hopefully soon and I hope SSA brings them in Tactical Loads!!!!!!!!:D

AKsarben
03-27-10, 08:34
Personally, I'd like to see someone come out with a 120 gr flat base bullet in a spire point. The reason for spire point is that it can be loaded in the case and not take up as much room. I don't think copper would be the best choise for a heavier bullet as you want heavy and dense, like lead, to cut down on the length of the bullet for 120 grains.

95 gr sounds good. Seems like so many times we seem to be going opposite of what the 6.8 was designed for. Keep getting lighter bullets and pretty soon you might just as well have a 5.56 NATO loaded with heavier bullets to begin with.

I would think that the 95 grain bullet would also be good for the 1:10 barrels like some of those out there, and the 1:10.5 that DEZ Arms makes.

LockenLoad
03-27-10, 09:26
Same reason LWRCI has stuck with 1/10--because 1/10 works just fine when shooting the factory loads that all military and LE folks use and the faster twist can be beneficial for shorter barrel weapons.

that's why I am not sweating having a 1/10 twist on my 8 inch psd, thanks Doc for educating me some more much appreciated.

AKsarben
03-28-10, 16:57
Doc,
Wouldn't you say that on the 6.8 SPC II (aka 6.8 X 43) that the rifling twist is important to the length of the bullet? I was reading a piece by Tire Iron about the early twist rates of 5.56 (topic here -> http://mscg.yuku.com/topic/1409?f=1 and one thing I did gather is that the twist rate is more important to bullet length for stabalizing than it is for bullet weight. A copper bullet will be longer for the same weight and might need that 1:10 twist to get better longer range stability, where a heavier shorter bullet might get by with 1:12 twist.

If the chamber of the 6.8 is cut correctly with the proper angle and leade in the throat, 1 in 10 or 1 in 10.5 might be considered to be a pretty good twist for longer copper Barnes bullets, or any Barnes copper bullets, over the 1 in 11 twist.

Food for thought.

Also the Greenhill formula: Greenhill Formula
Proper Twist = (150 X bullet diameter) / (bullet length / bullet diameter)

For the mesurements use calibers.

For velocities above 1800 use 180, in place of 150.

So for say a velocity of 2800 fps (thereabout) would that translate to (280 x .277) / (length/dia) ?

DocGKR
03-29-10, 15:24
1/10, 1/11, 1/12 all work fine for the 85-115 gr 6.8 mm bullets currently on the market. The faster 1/10 twist may be better for 10" and under barrels with some projectiles; the slower 1/11 & 1/12 twists work best for typical 16" barrels. Note the two co-designers of the 6.8 mm both have recommended a slower twist, 3 or 5 groove barrel...

BLACK LION
03-29-10, 19:23
Deja-vu???

BLACK LION
03-29-10, 19:28
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.

They mark 1-11 SPCII on the barrel... at least the barrel cut from a wilson blank did. I have yet to see if they mark the shillen barrels they use now.

CQC.45
03-30-10, 10:05
For those of you with Robinson Arms XCR rifles, they are now using the 1/11 twist rate with SPCII chamber and 4-groves in their 6.8 barrels. :)

Good to hear. I've been considering that caliber conversion.:)

LockenLoad
03-30-10, 13:05
[QUOTE=DocGKR;613951]1/10, 1/11, 1/12 all work fine for the 85-115 gr 6.8 mm bullets currently on the market. The faster 1/10 twist may be better for 10" and under barrels with some projectiles; the slower 1/11 & 1/12 twists work best for typical 16" barrels. Note the two co-designers of the 6.8 mm both have recommended a slower twist, 3 or 5 groove barrel...[/QU

I would think they might know best, unless extensive testing done later proves otherwise.

majohnson
03-30-10, 22:56
Just back from my 2 range trip shooting my Spec II with a 1/11 twist and 6 groves. Shooting a combo of Hornady 110gr Vmax factory and handloads. Even with the wind it shoots tight groups at 100 yds.

constructor
04-01-10, 01:03
I have already tested some 120gr bullets and they were flat based but I believe the final product will be a boattail, it shot fine out of the 12 twist 3 groove.

Personally, I'd like to see someone come out with a 120 gr flat base bullet in a spire point. The reason for spire point is that it can be loaded in the case and not take up as much room. I don't think copper would be the best choise for a heavier bullet as you want heavy and dense, like lead, to cut down on the length of the bullet for 120 grains.

95 gr sounds good. Seems like so many times we seem to be going opposite of what the 6.8 was designed for. Keep getting lighter bullets and pretty soon you might just as well have a 5.56 NATO loaded with heavier bullets to begin with.

I would think that the 95 grain bullet would also be good for the 1:10 barrels like some of those out there, and the 1:10.5 that DEZ Arms makes.

Artos
04-01-10, 17:53
Our stainless 1-11 4 groove bbls have so far proven to shoot extremely well and in some cases just as well as some big name bbls costing twice as much.


Bill, what is the lightest 6.8 tube you have on a carbine gas system?? I will be cutting it down to 11.5 or 12.5.

thanks,

Paul

Nevermiss
04-01-10, 22:48
I wanted a SBR to run suppressed and I picked up a Noveske 6.8 12.5" Crusader with switchblock and took it to the range on Monday. I mounted an Aimpoint T1 and it shot tight groups at 50 yards with SSA 110 gr Barnes TSX. I mounted my Leuopold 4.5-14X50 LRT just to see what it would do at 100 yards, despite a swirling 5-10 mph wind with crosswind gusts to 20 mph. My first 5 shot group measured 0.95" :D (from a 12.5" barrel!)

I took off the leupy and put the T1 back on. I'm awaiting my can. I hope the the 95 gr TTSX will group just as well.

I think I need to get a 16" barrel with lightweight upper for my daughter's first hunting rifle! (Can't hunt with a can in NC :()

rsilvers
07-17-10, 22:11
I ran some Miller twist values for some popular 5.56mm loads and a 6.8mm load.

http://accurateshooter.wordpress.com/2008/06/03/

A 5.56mm TAP 75 at 2800 fps is a 1.80 Miller value at 68F in a 1:7 twist barrel (I would say you always want to be 1.20 or more worst-case (low velocity and freezing temps)). Some believe (including me) lower values help ultimate precision and that is where varmint, benchrest, and Palma people tend to live.

The flip side, as was posted earlier, is that there is evidence that higher bullet RPMs increase terminal effects with some types of bullets. I believe this is real, so how high is too high for RPM?

Well, a 55 grain Speer TBBC, a bullet that does see increased terminal effect potential at higher RPM, is a Miller 4.02 at 68F at 2800 fps in a 1:7 twist barrel. So one can argue that, not for benchrest levels of precision, but perhaps for terminal effects, high RPM with Miller values up to 4 have a purpose in a combat, LE, or hunting weapon.

How does this map into 6.8mm?

Well, a 110 Barnes T-TSX (a long bullet) at 2600 fps and 68F is a Miller:

1:6.25 twist - 3.94
1:7 twist - 3.09
1:8 twist - 2.36
1:9 twist - 1.87
1:9.5 twist - 1.68
1:10 twist - 1.51
1:11 twist - 1.25
1:12 twist - 1.05

And it would seem like, for this bullet or longer, one would not want slower than 1:11 twist in any case - but especially from a short barrel or in colder temps. One could easily expect a Miller value of 1.05 for 1:12 twist with this long bullet to result in a baffle strike on a sound suppressor.

Now normally one could argue there is a point to even a 1:6.25 twist if you want the same Miller factor as a 5.56mm TBBC from a 1:7 twist barrel. The problem with that logic is that the TBBC is a short bullet for 5.56mm, and the TTSX is a long bullet for 6.8SPC - so selecting a 6.25 twist would be a bad idea across the board because then Miller values would go very high for shorter/faster bullets. So to select a twist, it might be a good idea to do a sanity check and pick a shorter 6.8mm bullet, such as the 90 grain. A 1:9 twist would give it about a Miller 4.01 at 68F and 2800 fps.

Therefore, to the best that I can calculate with the lightest popular bullet in each caliber, if you like 1:7 twist for 5.56mm, then you should like 1:9 twist for 6.8mm.

Specifically looking at 1:10 twist - as that has some controversy, it is an idyllic 1.51 at 68F. At -20F, it is still a stable Miller 1.26 - and even at 2200 fps from a short barrel is still an acceptable Miller 1.19.

So I could make a case for anything from 1:8.75 to 1:11, depending on the bullet, barrel length, and if you wanted ultimate precision or ultimate terminal effects with certain bullets. But to say 1:10 is obsolete and you need to avoid it - that is just not the case and it is a perfectly reasonable choice. Just like 1:12 is a reasonable choice for those who believe in the benchrest shooting or Palma philosophy of spinning as slowly as possible under match conditions.

That being said - one should pay close attention to lands and grooves, bore cross sectional area, and chamber dimensions.

M193 did not blow up rifles when the military went from 1:12 to 1:7. Some people predicted gloom and doom, but it never happened. And if people went to 1:9 twist in 6.8mm (with a good chamber and bore design), there would also be no drama.

AKsarben
07-18-10, 00:58
In practice it doesn't work out that way. They, Art Kalwas, Harrison Beene, et al. have done pressure tests on barrels with different twists and chambers, IE 6.8 SPC II (leade of 0.100" instead of 0.050" ), and have determined that a 1:13 twist can stabilize most bullets in what the 6.8 SPC will shoot except for the longer/heavier 130 grain bullets, but 1:12 had no problem with getting all bullets stabilized with what was useable for the 6.8 SPC. If you want to see a lot higher pressures, then 1 in 9 is a way to increase pressure and do nothing to the stability nor accuracy of the particular cartridge. a 1:10 is about as fast a twist as you want, and then it's greates contribution is in the shorter 14" and less barrels.

rsilvers
07-18-10, 11:25
In practice it doesn't work out that way. They, Art Kalwas, Harrison Beene, et al. have done pressure tests on barrels with different twists and chambers, IE 6.8 SPC II (leade of 0.100" instead of 0.050" ), and have determined that a 1:13 twist can stabilize most bullets in what the 6.8 SPC will shoot except for the longer/heavier 130 grain bullets, but 1:12 had no problem with getting all bullets stabilized with what was useable for the 6.8 SPC. If you want to see a lot higher pressures, then 1 in 9 is a way to increase pressure and do nothing to the stability nor accuracy of the particular cartridge. a 1:10 is about as fast a twist as you want, and then it's greates contribution is in the shorter 14" and less barrels.


Those who favor 1:9 twist barrels in .223/5.56mm also point out it will stabilize most common bullets and going to a faster twist will raise pressure and do nothing for stability or accuracy.


I would like to give a detailed response to both of your points of stability and pressure. First, I will address the issue of stability.

A real world test is always better than simulation, but only if the test is set up to show the worst-case one might encounter - and then be is able to define and detect what is passing criteria. It is possible that 1:12 is fine for all common bullets, but I don't see how the test report you are referencing establishes that. First because their testing was done at 103 F which makes bullets more stable, and second because they did not have a way, such as Doppler RADAR or high speed video, to be able to detect when the bullet is not stable close to the muzzle (Because linear velocity drops off faster than rotational velocity, the least stability is close to the muzzle - where a sound suppressor baffle strike might happen).

http://68forums.com/forums/showthread.php?t=7220

Just to show how temperture effects stability, a 110 grain projectile which has a Miller value of 1.31 at 103 F and 2600 fps (which would be expected to look fine on paper) changes to 1.07 at 0 F - which may cause a sound suppressor baffle strike some percentage of the time. And 130 grain bullets, which are both lower in velocity and require a faster twist to achieve the same stability factor, would have even lower margins of safety.

It may not be practical for anyone but the military to test at -20 F, but something below freezing is pretty doable. To establish worst case on velocity, you would find the average muzzle velocity from the shortest barrel you are interested in with the gun and ammo having been stored in an environmental chamber at below freezing temps. You would than need to measure the velocity standard deviation, and multiply that by 3, and subtract it from the average. That is then the velocity to use for the Doppler traces or high speed video.

Simulations are just a tool, but they are designed to correlate to real-world results that are sometimes otherwise difficult to measure. For example, if one were desining an elevator cable and a simulation showed a 1.05 factor of safety, a real-world test of it holding the actual weight would not be expected to show a failure - but that does not mean the cable is ok. One should still design in a higher factor of safety. Of course this needs to be balanced against cost. What is the cost of a thicker elevator cable in dollars and weight? What is the cost of going to 1:11 twist in dollars ($0) and pressure? I believe the cost of 1:11 is very low.

AKsarben
07-18-10, 12:00
One barrel maker, very few know about DEZ Arms www.dezarms.com sell a 6.8 SPC II chambered barrel with CMV steel and a twist rate of 1 in 10.5 inch. Kind of not 1:11 and not 1:10, but in between. Their barrels are also not prohibitively expensive either.

rsilvers
07-18-10, 20:24
What chamber pressure are SSA Combat loads when tested using the SAAMI protocol in a minimum dimension SAAMI chamber and in a minimum dimension SPC-II chamber? We really should know that before interpreting the results of the paper.

Related to twist and pressure:

There is no evidence that shows 1:10 twist, independent of other variables, significantly increases chamber pressure. And we do know that, going from 1:12 to 1:7 twist for hot 5.56mm M193 NATO ammo did not cause a problem.

Those thinking 1:10 twist is too fast for 6.8 often cite this paper but it does not isolate concerns related to twist for two reasons:

1. None of the tests compare barrels that only differed in twist.
2. Even if they did, strain gauges are not reliable for comparing one barrel to another and are best left for relative pressure changes on the same barrel.

Will faster twist make more pressure? All else being equal it should, but it should be a very small amount. Yes, it is intuitive that spinning a bullet faster takes energy, but it is likely much less significant than the effect of the cross sectional bore area and specific chamber dimensions. For example, were the reamers for each barrel at minimum spec or at min + 0.0015?

The author has an opinion that twist is best from 1:11 to 1:13. Using as little twist as possible is something that a lot of shooters believe in – especially for benchrest or Palma shooting. So there is plenty of precedent for that. However, there are other schools of thought to consider. For example, the 1:7 twist popular in 223/5.56mm comes from wanting as much flexibility as possible and may also benefit from the increased terminal effects of higher rotational velocity with certain bullets. Concerns over being ‘too fast a twist’ have faded because the results are satisfying.

Labs which compared the chamber pressure of 223/5.56mm 1:12 and 1:7 twist have reported from no to minimal differences. I have also seen data comparing velocities from otherwise identical 308 barrels, made from the same reamer, and only different in 1:8 vs 1:12 twist. There was no difference. Some have said that the 6.8mm is a special case and cannot be compared to 223 and 308. That may be true (doubt it), but that would be an unusual and unexpected result and so one would need a real test to prove it (using a conformal or case-mouth transducer, not a strain gauge – and changing nothing but twist between barrels).

Because SAAMI proof pressure is about 78,000 psi, and one should not be loading 6.8mm over 60,000 psi, the likely few hundred or less psi difference between 1:10 and 1:11 twist is not significant. Still, an SPC-II chamber would be a good thing as that is just good design for this application.

After running Miller twist values, it would seem like 1:8 and 1:13 twist are the outer fringe for the typical bullets used for magazine-length ammo in the 6.8 SPC. If you are from the “Use a 1:7 twist for 223/5.56mm” school of thought, then 1:9 twist for 6.8mm seems to be the closest mapping. If you are from the “Use the slowest twist that will make most common bullets stable nearly all of the time because I am trying for benchrest levels of precision” then 1:12 seems like the way to go. 1:10 and 1:11 are right in the middle and moderately biased toward each school of thought.

rsilvers
07-25-10, 15:34
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

The 130 Barnes is 1.280 inches long.
The 140 Barnes is 1.300 inches long.
The 110 AccuBond is 1.105 long.
The 130 AccuBond is 1.245 long.
The 140 AccuBond is 1.310 long.

For temp, I like to put in the coldest I might shoot. For the military, that might be -40 or -20F. For me, it is about 0F.

For velocity, you should use your average velocity minus 3 times the standard deviation. So if you are at 2600 with a deviation of 15, then use 2550. That is because one out of every 280 or so shots will be 3 standard deviations below average. Basically subtracting 50fps from the average should cover this.

Velocity of a 130/140 bullet may only be 1950 fps from an 8 inch barrel in the cold for some shots. This is showing that a 1:9 twist should cover things for that application.

BSHNT2015
07-30-10, 10:41
Lots of good info here, thanks for sharing.

constructor
08-07-10, 14:34
Most of the 10 twist barrels on the market have a land to groove ratio between 50:50 and 40:60 which does increase pressure over the 30:70 and 25:75 ratio barrels. If someone actually made a 10 twist with a 30:70 ratio then it may not show much more pressure than a 11 twist with the same ratios. Until someone does the 11-12 twist barrels with the correct ratios are a better choice.

constructor
08-07-10, 14:39
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

The 130 Barnes is 1.280 inches long.
The 140 Barnes is 1.300 inches long.
The 110 AccuBond is 1.105 long.
The 130 AccuBond is 1.245 long.
The 140 AccuBond is 1.310 long.

For temp, I like to put in the coldest I might shoot. For the military, that might be -40 or -20F. For me, it is about 0F.

For velocity, you should use your average velocity minus 3 times the standard deviation. So if you are at 2600 with a deviation of 15, then use 2550. That is because one out of every 280 or so shots will be 3 standard deviations below average. Basically subtracting 50fps from the average should cover this.

Velocity of a 130/140 bullet may only be 1950 fps from an 8 inch barrel in the cold for some shots. This is showing that a 1:9 twist should cover things for that application.

None of those will fit in a mag

rsilvers
08-08-10, 22:47
None of those will fit in a mag

I think you use the 110 Accubond.

As for the others, I don't have drawings for them, but I checked some other bullets.

Sierra 150 GK, 1.258 long.
Deepest one can seat this bullet: .651
Amount of bullet in case when loaded to 2.283 OAL: 0.657

Berger 150 VLD, 1.310 long.
Deepest one can seat this bullet: .694
Amount of bullet in case when loaded to 2.283 OAL: .683 (0.011 off but close enough).

On the other hand, these don't seem to be a match (when loaded to 2.283" but some may work if you are able to load longer):

Berger 130 VLD, 1.181 long.
Deepest one can seat this bullet: .560
Amount of bullet in case when loaded to 2.283 OAL: .603

Berger 140 VLD, 1.245 long.
Deepest one can seat this bullet: .627
Amount of bullet in case when loaded to 2.283 OAL: .643

Sierra 135 MK, 1.250 long.
Deepest one can seat this bullet: .524
Amount of bullet in case when loaded to 2.283 OAL: .707

Nosler 140 Partition, 1.195 long.
Deepest one can seat this bullet: .573
Amount of bullet in case when loaded to 2.283 OAL: .597

Hornady 130 Interbond, 1.262 long.
Deepest one can seat this bullet: .544
Amount of bullet in case when loaded to 2.283 OAL: .671

Hornady 140 BTSP Interlock, 1.215 long.
Deepest one can seat this bullet: .606
Amount of bullet in case when loaded to 2.283 OAL: .625

Hornady 150 SP Interlock, 1.220 long.
Deepest one can seat this bullet: .656
Amount of bullet in case when loaded to 2.283 OAL: .782

So going back to twist... If you wanted to be able to shoot the 150 VLD at subsonic speeds at very cold temps (worst case), then a 1:8 twist should do it with a 1.3 stability factor.

If you wanted to shoot that bullet at around 1800 fps at cold temps, say in an LWRC PSD, then 1:9 twist would give a 1.4 stability factor.

So probably with an 8-9 inch PDW type of setup, 1:9 would have been better then 1:10 if you wanted to be able to shoot any supersonic ammo with a good margin of safety. Or 1:8 if you wanted to be able to do subsonic ammo well.

Think of these long and not-so-common bullets as like using a Sierra 77-80 in a .223 rifle. It seems like the best analogy to a 1:7 5.56mm rifle is a 1:9 6.8mm rifle.

If you want to take advantage of the increased terminal performance of some bullets with faster-twist, then err on the side of faster twist. I am going to test that out. I have a 1:4 twist 308 barrel on order.