[Drifter]

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.

Thanks

 

[Cold]

The ideal twist for the 6.8 is anything from 1:11 to 1:12

 

[jrswanson1]

I just bought a DPMS upper in 6.8 and it has 1 in 11".

Jim

 

[marinesg1012]

Have you read the testing report?
http://www.yuntaa.com/FileManager/D...d=&ContentID=56293A63F2F3475CE04400144FB7B71E

Basically the slower twists reduce preassure which allow you to load the ammunition hotter (or back to the same preassure as the slower twist) which will give you increased terminal range...

 

[Drifter]

Basically the slower twists reduce preassure which allow you to load the ammunition hotter (or back to the same preassure as the slower twist) which will give you increased terminal range...

Is less pressure the only benefit of the slower twist?

I understand twist in general and how it's tied to bullet length / stabilization, but looking for practical difference between rifles with 1:10 twist versus 1:11 twist using factory ammo.

Is the twist less relevant concerning pressure when using factory loads?

Are rifles with 1:10 twist okay, or should they be completely avoided?

Thanks for the replies so far.

 

[paulosantos]

Is less pressure the only benefit of the slower twist?

I understand twist in general and how it's tied to bullet length / stabilization, but looking for practical difference between rifles with 1:10 twist versus 1:11 twist using factory ammo.

Is the twist less relevant concerning pressure when using factory loads?

Are rifles with 1:10 twist okay, or should they be completely avoided?

Thanks for the replies so far.

A 1:10"/6-Groove/SAAMI barrel will be able to handle nearly all of the factory ammo. So if you already have one, I'd convert it to the SPCII chamber and enjoy. If you don't have a barrel yet, I'd get a barrel with at least the 1:11"/4-Groove/SPCII chamber.

The more you reduce the pressure, the longer your brass will last and it puts less wear and tear on your upper.

 

[JAOwens]

Is less pressure the only benefit of the slower twist?

I understand twist in general and how it's tied to bullet length / stabilization, but looking for practical difference between rifles with 1:10 twist versus 1:11 twist using factory ammo.

Is the twist less relevant concerning pressure when using factory loads?

Are rifles with 1:10 twist okay, or should they be completely avoided?

Thanks for the replies so far.

Most 1/10 rifles are gtg with any factory load available, IF, they have a spc II or DMR style chamber with the .0100 lead. As an example my older stag 5h with spc II and 1/10 twist will shoot any factory load out there. But the chamber is a VERY important part of the equation, as many of the older 1/10's had SAAMI spec chambers and could start exhibiting pressure signs with most commercial loads hence why Remington 6.8 ammo is rather anemic.

That being said If you handload or want to start handloading ion the future a 1/11 twist with the proper chamber will allow you much more lee way and will allow a significant increase in velocity

 

[Drifter]

Thanks for the additional replies.

I use factory ammo only, so might try an RRA 6.8 upper with 1:10 twist. Don't really want an extended wait to get something rea$onable with 1:11.

 

[CaseyJ]

I have not searched this yet, mainly because 6.8 is new to me, but would gain twist rifling help the pressure problem even further and maintain the accuracy?

 

[jough]

Thanks for the additional replies.

I use factory ammo only, so might try an RRA 6.8 upper with 1:10 twist. Don't really want an extended wait to get something rea$onable with 1:11.

Depends which factory ammo. You "might" be ok with the SSA hotter loads and SPECII / 1 in 10, but then again, you might not be ok. Do the research, get it right the first time. You won't be disappointed......and you've found the right place to learn about 6.8.

 

[Drifter]

Depends which factory ammo. You "might" be ok with the SSA hotter loads and SPECII / 1 in 10, but then again, you might not be ok. Do the research, get it right the first time. You won't be disappointed......and you've found the right place to learn about 6.8.

I have a pretty good supply of 6.8 "commercial" ammo (mostly SSA) for my Mini-14 6.8 SPC. Just wanting a decent AR upper (again, without an extended wait) for whitetail hunting from stands, with shots likely to be 300 yards or less.

I can accept that performance will not match "combat" and / or handloads from 1:11. And not looking for to do high-volume shooting with an AR 6.8.

Just didn't want to get a 1:10 twist if y'all had determined it to be no good. From the responses, seems like 1:10 is not optimum, but still okay.

To steer my own thread off topic, would a barrel change be feasible in the future if I go with a completed RRA upper for now?

 

[colt100]

I have a pretty good supply of 6.8 "commercial" ammo (mostly SSA) for my Mini-14 6.8 SPC. Just wanting a decent AR upper (again, without an extended wait) for whitetail hunting from stands, with shots likely to be 300 yards or less.

I can accept that performance will not match "combat" and / or handloads from 1:11. And not looking for to do high-volume shooting with an AR 6.8.

Just didn't want to get a 1:10 twist if y'all had determined it to be no good. From the responses, seems like 1:10 is not optimum, but still okay.

To steer my own thread off topic, would a barrel change be feasible in the future if I go with a completed RRA upper for now?

Dude, Save yourself the headache and get an extreame. They are catching up real fast and from what I have been reading, will have all their orders filled around the end of sept. They have some of the best uppers for the money out there right now. You are basically getting a match, combat upper for the price of a standard put together upper from RRA, Bushmaster, Ect and the extreame will have the correct specs to be able to shoot any ammo you will want to out of it.

Oh, and they are sponsers of this site also.

 

[ArtFWTx]

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.

http://m.b5z.net/i/u/6132121/i/6.8_20SPC_20Performance_20Testing_20Report_1_.pdf

 

[HTR]

What has been found, through the various reports done by Constructor, Tim W and me, is that there are three factors that affect pressure to different degrees in the 6.8 X 43 SPC.

1) the most important seems to be the chamber. The freebore of the SPC II chamber allows for the greatest velocity because you can get more powder in a case before pressure signs are exhibited.

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.

3) with # 1 and 2 being equal, the twist rate then does seem to make some difference. So, if you have two 5R barrels and one is 1:10, the other 1:11, you might see a very slight difference in pressure handling from the faster to slower twist.

*** don't forget, if you prefer to shoot long 130 grain bullets or 135 grain SMK's, for whatever reason, you would need the 1:10 twist, as the rules of bullet stability still apply. That having been said, a 11 or 12 twist can still stabilize most 130 grain projectiles. However, most of us are seeing that 80- 115 grains gives the best velocity and overall performance with supersonic rounds. I personally just bought a 1:10 twist barrel to test 135, 140 and 150 grain subsonics.

For all practical purposes, you should not have to wait very long for an excellent gun with good specs. AR Performance, Noveske, Bison Armory and WOA all make great 6.8 rifles, and should have them available within a reasonable time frame.

 

[ArtFWTx]

"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.

 

[Lastrites]

Another do your self a flavor, forget the originals and go for what current or better. SPCII/DMR + 11 twist, as most of the AR companies have come to their sense's and converted. Sometimes it takes a while to smell a rose.

 

[tbready]

Another do your self a flavor, forget the originals and go for what current or better. SPCII/DMR + 11 twist, as most of the AR companies have come to their sense's and converted. Sometimes it takes a while to smell a rose.

Yea, kinda like LMT. Maybe we need to gang up on someone over there. Would like an updated barrel for my MRP as well as many others.

 

[Test_User]

The question was asked: What if any other benefits come from using a slower twist other than reduced pressure. There are a few which I have posted before but might not have come up with a twist search.


One is bullet trajecory. The faster a bullet is spun the more it tends to stay at the same angle as it leave the muzzle this means that are it starts down the back slop of the traj arc the bullet tip will want to stay tip up instead of following the traj path of downward angle. this will put air turbulence on the side of the neck and body and off the tip whihc can upset the bullet and reduce accuracy.

The other benefit of using slower twist whihc I wrote a paper on which I can not seem to find but was also substantiated by a mil report is increase or decrease in throat erosion. The slower the twist the slower the throat and bore erosion. The fast the twist the more erosion. The reason for this is the sharper angle the rifling is that the bullet hits at the end of the throat the more stress it places not only on the bullet jacket but alos the lands themselves. The bullet actually skids at eh every begining of the lands whihc increases the wear on those lands. The navy plotted barrel erosion over the transitions from 12 eventually up to 7 twist barrels and they found there was a direct correlation between the increased twist and throat erosion and growth.

Bullet integrity. This may not be an issue with the bullets used in the 6.8 presently but when thinner jacket such as the J4s are used with faster twist significantly more stress is placed on the jacket . In 6 and 6.5 mm there has been many that have had bullet jacket fail becasue of this causing the bullets to disintegrate as they travel downrange. Given we are now pushing bullets well over 3000 even from 16" barrels such as the Xtremes this could become an issue.

LEss velocity. Not only does slower twist seem to lower pressure but it also seems to increase velocity even with the lower pressure as there is less reistance as it goes down the bore it makes the charge more efficeitn. Takes less powder or peak pressrue to achieve the same velocity.

 

[Bull000]

I don't really shoot hot loads..... yet. When i eventually get a reloading station set up and start playing with this, i will move up to a 1:11 or 1:12 twist. Probly buy an XTreme sometime next year. But as far as shooting my RRA with a 1:10 SPC II barrel? I can split boulders in half out in Del Rio with 110 Pro Hunters... so i think i'm happy. Still haven't gotten on any hogs with it yet, and i've had it a whole year. This year though... I have a couple places to hog hunt so hopefully i'll have some pics for ya'll.

Like i said though, i bought mine last year without knowing. But i'm very happy with it. From now on though, The Pros on this website are highly reccomending the 1:11 twist/SPCII/4 Groove (with M4 Feed Ramps on your reciever), and i would listen to them. Buy an TXtreme from AR performance : ) I know i will once i save up the money. Either that or an Armalites when they have all of these features. Don't think they are out yet.

But a RRA with 1:10/SPCII barrel is still a good rifle for the price, but i wouldn't shoot any handmade hot laods at the max out of mine. I believe they will be moving to 1:11 soon enough, just like all the others.

 

[paulosantos]

I don't really shoot hot loads..... yet. When i eventually get a reloading station set up and start playing with this, i will move up to a 1:11 or 1:12 twist. Probly buy an XTreme sometime next year. But as far as shooting my RRA with a 1:10 SPC II barrel? I can split boulders in half out in Del Rio with 110 Pro Hunters... so i think i'm happy. Still haven't gotten on any hogs with it yet, and i've had it a whole year. This year though... I have a couple places to hog hunt so hopefully i'll have some pics for ya'll.

Like i said though, i bought mine last year without knowing. But i'm very happy with it. From now on though, The Pros on this website are highly reccomending the 1:11 twist/SPCII/4 Groove (with M4 Feed Ramps on your reciever), and i would listen to them. Buy an TXtreme from AR performance : ) I know i will once i save up the money. Either that or an Armalites when they have all of these features. Don't think they are out yet.

But a RRA with 1:10/SPCII barrel is still a good rifle for the price, but i wouldn't shoot any handmade hot laods at the max out of mine. I believe they will be moving to 1:11 soon enough, just like all the others.

Enjoy the RRA. It is a nice upper. The hogs aren't going to know that they were shot with a 1:10"/6-Groove/SPCII.