Range Report Suggestions on rifling twist for 6.5mm bullets

[McGuyver]

Hi guys,

just thought I'd post this question here to see what you all thought. I am looking to get a barrel on order for a 6.5 mm chambering to push some heavier 142 SMKs or 140 Berger VLDs. Looks like I've got three twist rates to choose from. 1:8, 1:8.5, and 1:9. The 1:9 looks like it is too slow for the heavy bullets. And I don't want to over stabilize if I don't need to, and I do live at higher altitude (above 4400 ft) so I could probably do with less twist than some living near sea level. So with that in mind, I'm thinking a 1:8.5 would work. What do you all think?

edit: btw, I'm also looking for suggestions on barrel contour and length. Will be shooting F-class and practical/tactical with some possibility of (but not a lot of) carrying on a sling in the mountains. Thanks.

 

[Graham]

Re: Suggestions on rifling twist for 6.5mm bullets

There's no such thing as 'over stabilizing' a bullet. Either the bullet is spun fast enough to be stable, or it isn't.

8-twist. 24" - 26" for your purpose. Consider a light or med Palma.

 

[KNIGHT11B4]

Re: Suggestions on rifling twist for 6.5mm bullets

Im running a 1/8 26" Tubb contour.... And I love it. Shooting the 142 right now.

 

[Rob01]

Re: Suggestions on rifling twist for 6.5mm bullets

8 twist and 26 inches. The 8 twist I have on my 6.5 Creedmoor is perfect for the 140 amax I shoot.

 

[wnroscoe]

Re: Suggestions on rifling twist for 6.5mm bullets

If running a 6.5x284 or larger than a 1-8.5 would be OK. If running the smaller cartridges like 6.5x47L, 6.5CM, 260 etc. I'd run a 1-8 for the 139/140/142's. 26" would be my length.

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Thanks for the input guys. I haven't got an exact chambering figured out, but I am considering 6.5x55 Swede or 260 Rem. I'm going to respectfully disagree with graham on the overstabilization point. From what I recall benchrest shooters try not to stabilize more than what is needed because it can cause imbalances in the bullet to be magnified and hurt accuracy. Be that as it may, sounds like the 1:8 twist is the favorite here, which is what I was trying to determine. Question: what does the chambering have to do with bullet stability?? Everything I've learned seems to indicate that bullet weight and length are what is important (among a few other things).

 

[Boomholzer]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: McGuyver</div><div class="ubbcode-body">Thanks for the input guys. I haven't got an exact chambering figured out, but I am considering 6.5x55 Swede or 260 Rem. I'm going to respectfully disagree with graham on the overstabilization point. From what I recall benchrest shooters try not to stabilize more than what is needed because it can cause imbalances in the bullet to be magnified and hurt accuracy. Be that as it may, sounds like the 1:8 twist is the favorite here, which is what I was trying to determine. <span style="font-weight: bold">Question: what does the chambering have to do with bullet stability?? Everything I've learned seems to indicate that bullet weight and length are what is important (among a few other things). </div></div></span>

It takes energy to impart spin on a bullet. More spin = slightly removed muzzle velocity.
Chambering = bullet acceleration to muzzle velocity. Less velocity in the same twist-rate = less imparted twist. Another minor consideration for twist rate is barrel length.

 

[SVG308]

Re: Suggestions on rifling twist for 6.5mm bullets

As stated, go with an 8 twist. Perfect for the 140/142 weight bullets. I'd go 26-28" on barrel length.

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KNIGHT11B4</div><div class="ubbcode-body">Im running a 1/8 26" Tubb contour.... And I love it. Shooting the 142 right now. </div></div>

I've never heard of a Tubb contour. Is that something like a Palma? I was thinking of something like an MTU, a heavy target, or varmint. here's a link that gives descriptions:

http://www.kriegerbarrels.com/RapidCat/c...;CompanyId=1246

What do you think? I'm sure these would do well enough in competition, but am I going to really regret it if I decide to carry it up a mountain?

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Boomholzer</div><div class="ubbcode-body">It takes energy to impart spin on a bullet. More spin = slightly removed muzzle velocity.
Chambering = bullet acceleration to muzzle velocity. Less velocity in the same twist-rate = less imparted twist. Another minor consideration for twist rate is barrel length.
</div></div>

I agree with what you are saying about more spin requires more energy, thus slightly less muzzle velocity. However I was just considering twist from a bullet stability stand point. Miller's basic formula for bullet gyroscopic stability Sg is directly proportional to weight, inversely proportional to twist squared, and diameter and length cubed. Granted there are velocity and atmospheric correction factors as well. The velocity correction factor is directly proportional to the cube root of the muzzle velocity. <span style="text-decoration: underline">Is this why you bring up chambering and barrel length?</span> I suppose based on this last point, with <span style="font-style: italic">all other things being equal</span>, higher velocity will result in a higher gyroscopic stability; although this seems to be a minor contributor since it is proportional to the cube root of the velocity.

 

[bowslngr]

Re: Suggestions on rifling twist for 6.5mm bullets

1:8.5 is enough. I am running that in both 260 and 6.5 CM and get terrific results with the 139-142s.

That said, if I had both to choose from at equal quality and price I would take the 1:8.

 

[KNIGHT11B4]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: McGuyver</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KNIGHT11B4</div><div class="ubbcode-body">Im running a 1/8 26" Tubb contour.... And I love it. Shooting the 142 right now. </div></div>

I've never heard of a Tubb contour. Is that something like a Palma? I was thinking of something like an MTU, a heavy target, or varmint. here's a link that gives descriptions:

http://www.kriegerbarrels.com/RapidCat/c...;CompanyId=1246

What do you think? I'm sure these would do well enough in competition, but am I going to really regret it if I decide to carry it up a mountain? </div></div>

Here is a bit of info on it...

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1345500

 

[Cape_hunter]

Re: Suggestions on rifling twist for 6.5mm bullets

There is no such thing as over-stabilization.
The actual issue is the RPM of the projectile.
Where we get into trouble is continuing to use the term "over" stabilize.
The word stabilize is defined as follows:
1.to make or hold stable, firm, or steadfast.
2.to maintain at a given or unfluctuating level or quantity:
3.Aeronautics . to put or keep (an aircraft) in stable equilibrium, as by some special device.

How does one over stabilize anything? Well you dont.
In regards to a bullet it is ether stable or not. That being said you can however spin so fast it will become unstable.

If you spin it too fast the nose of the projectile does not follow the arc/path and can be high down range. Think the wheel on a bike. The other thing that can happen is a super fast spin can magnify any imperfections in the projectile.

Then there is also the spinning them apart thing. If this is the case then one could come to the conclusion that if a bullet will come apart there is a point where it may just deform and not actually come apart. I would think that would hurt your accuracy.

 

[BGE541]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">There's no such thing as 'over stabilizing' a bullet. Either the bullet is spun fast enough to be stable, or it isn't.

8-twist. 24" - 26" for your purpose. Consider a light or med Palma. </div></div>

You can run into other issues. Granted a high amount of gyroscopic torque can more so effect a round at long, long ranges via "steering" shich is ultimatly in all bullets, but will grow in the case of a "faster RPM" There has to be a balance between dynamic stability and gyroscopic torque, if you have a very long bullet (say JLK or VLD) you can have a lack of dynamic stability (requiring more gyroscopic torque). Yet if you run a fast twist and try to push a little bullet out of it, it can and will effect accuracy or even explode do to too high of RPM's. (Note to high of gyroscopic stability, jackets shreding) Now on all my 6.5s I run a 1:8tw. Ones a 20", a 24" and a 26". Which casing you gonna go with? Go 1:8 and youll be good, but when yo get to really pushing things fast you need to worry about in this case. Go cut rifled barrel, look at BugHoles on here or others, they offer good stuff.

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Cape hunter, Graham,

You are of course correct in the literal sense of the word, however when I say "overstabilized" I'm being loose in referring to the gyroscopic stability factor Sg (because it's quicker and easier to say). Too high of an Sg can lead to accuracy issues (already discussed), tractability problems, and if high enough, (iirc) can lead to dynamic instability and ultimate failure of the bullet due to centrifugal forces tearing it apart as was also already mentioned above. For all of these reasons, it is prudent to avoid spinning the bullet too much faster than what is needed for gyroscopic stabilization.

 

[Cape_hunter]

Re: Suggestions on rifling twist for 6.5mm bullets

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: McGuyver</div><div class="ubbcode-body">Cape hunter, Graham,

You are of course correct in the literal sense of the word, however when I say "overstabilized" I'm being loose in referring to the gyroscopic stability factor Sg (because it's quicker and easier to say). Too high of an Sg can lead to accuracy issues (already discussed), tractability problems, and if high enough, (iirc) can lead to dynamic instability and ultimate failure of the bullet due to centrifugal forces tearing it apart as was also already mentioned above. For all of these reasons, it is prudent to avoid spinning the bullet too much faster than what is needed for gyroscopic stabilization.</div></div>

Sounds as though we are all on the same page.

 

[Greg Langelius *]

Re: Suggestions on rifling twist for 6.5mm bullets

I'm with WNRoscoe on this one. We can theorize, calculate, and debate on this until well past dark; but his approach is besed on basic experience. Listen to the man...

I like the .260 but would probably like the Swede's small additional capacity more. I just haven't seen a precision rifle chambered in the Swede that's quite up to the accuracy I've seen with the .260. That's probably more a matter of which particular rifles I've been watching. That's one reason why I'm sorta investigating the .280 Rem's potential, but I'm not really going about the task with true fervor.

Me; I'd try the Swede with an 8.5" twist and a 26" barrel, and give serious thought to an even slower twist with bullets exclusively in the 120gr and under range. It's clearly possible to shoot 120's to 1Kyd wth the .260; but ya gotta get 'em movin' right quick, and they do strange things for me 'way out there with the 1:8" twist...

For instance they appear to keyhole, but the impact prints all appear to be oriented nose-up, and the drops are way more than what's calculated. If I had to, I'd say they might even be representing the elusive 'overstabilized bullet'.

Greg

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Hi Greg,

I agree it sounds like a tractability problem caused by a bullet spun too fast (what I was referring to previously as "overstabilized"). The idea is that the bullet axis does not track the changing trajectory angle, (think of a football in flight) due to the axial rigidity imparted by the high rate of spin and you end up with a large yaw angle at the end of trajectory that increases drag, and of course that key hole shape on the target that you mentioned.

However I'm skeptical about this being the explaination for what you are seeing because I understand this phenomenon to only to be a problem on projectiles with high launch and terminal angles like artillery, and most small arms only practically use relatively flat fire trajectories.

 

[Greg Langelius *]

Re: Suggestions on rifling twist for 6.5mm bullets

We share the same mind's eye view. I can imagine it, but it's hard to prove.

But the results are consistent; what appears to be a nose up yaw out at the very end of a plunging trajectory.

I am sorely tempted to try a slower twist and see what happens. This project is on my list, but requires a test barrel.

Time and trial heals all SWAGs.

Greg

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Yes, as they say, the proof of the pudding is in the eating.

 

[nesikabay]

Re: Suggestions on rifling twist for 6.5mm bullets

Why not a gain twist with a taper bore. Finish at 8 twist. A taper bore is nice.

 

[jake6547]

Re: Suggestions on rifling twist for 6.5mm bullets

1:8 is the most universal twist rate for the smaller 6.5 calibers. 1:8.5 for above 2950fps for the 139,140,142g

 

[Seth8541]

Re: Suggestions on rifling twist for 6.5mm bullets

I would suggest a 1 in 8. It'll do you well for the lighter bullets as well as the heavier 140 class bullets. I'm running a 30" Kreiger 1 in 8.5, 39.3gns of RE-17 and SMK 142's. Has an issue stabilizing the 142's @ 100 yds. It'll shoot .5" 5 round groups all day at 100 yards. Frustrated the hell out of me. Spent 300 rounds doing load development trying to get it to tighten up. Then I said to hell with it and started shooting past 100. Funny thing is, it shoots .3" groups @ 300 and .5-.75" groups at 400 yards with no wind. After seeing that I called my load good, and stopped shooting at 100. If i dropped down to the 130 gn pills I'm sure it'd hammer at 100.

Go with the 1 in 8 and call it good.

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Thanks guys, I think I will go with the 1:8 so I can push a 140 Berger VLD without worry. Not decided on whether to chamber in .260 Rem or 6.5X55 Swede yet, but the ballistics are comparable enough that I don't think the answer to that question will matter much for twist rate. Not decided yet on contour and length. Thinking of an MTU type contour - such as M24/M40, varmint (light or heavy?) maybe a standard or heavy target? This will be used on an F-class and/or tactical/practical shooting setup. Considering barrel lengths from 24" to 28"? What are your thoughts?

 

[KNIGHT11B4]

Re: Suggestions on rifling twist for 6.5mm bullets

Might as well go 260 since components and parts are easier to find. As I said before I would go with the Tubb Contour.

AS far as barrel length I would go 26. Some really good guys run comps with a 28 but I like the 26. It would still perform well on a F-class range.

 

[McGuyver]

Re: Suggestions on rifling twist for 6.5mm bullets

Who does the Tubb contour? I don't see it offered by Krieger or Bartlein.

 

[Frank Green]

Re: Suggestions on rifling twist for 6.5mm bullets

We do the Tubb contour.

Just because a contour is not listed does not mean we don't do it.

Picture if you could how long the contour page would be if we listed everything we've done or can do!

We can do most contours at no extra charge. Really special contours will get a extra charge to them but as long as it's a more along the lines of normal things being done it's no big deal.

A Tubbs contour is a modified Rem. Varmint contour basically.

We've done barrels nearly 60" long but you don't see that listed on the website either.

Any questions let us know.

Later, Frank
Bartlein Barrels

 

[ChadTRG42]

Re: Suggestions on rifling twist for 6.5mm bullets

I run a 1:8.5 twist on all my 260 Rem barrels. It will stabilize all the 140 class bullets just fine. Technically, a 1:9 will also stabilize a 140, but it's right on the edge.

The only thing I've seen an issue with is shooting my 260 Rem 160 grain RN subsonics. The 160's out of a 1:8.5 twist have a slight oblong hole on paper at 100 yards, and the 8 twist is nice and round.