I may have missed the boat!

[M14AMU]

All the years I shot Palma I was very often considering the fact that instead of 1-13 or 1-14" twist barrels that I was using that instead they should have been 1-10" twist. It just made/makes sense to me that when shooting to 1000 yards that a good stability factor is much required! Even though I did well in my shooting I recently read about John Whidden's success using a 1-10" twist in his Palma rifle shooting the 155 Berger bullets. I have shot 150 gr. Nosler AccuBond bullets for years out of a very good .308 rifle with 1-10" twist and the accuracy has always been superb at the extended ranges! I feel as if.....I/we may have slightly missed the boat!



Extraction from John Whidden's article:

An Unconventional Twist Rate for a Palma Rifle

The most unusual thing about this rifle is the 10-twist barrel. Conventional Palma wisdom is to run a slower twist rate, such as 1:12″ or 1:13″. Tim North at Broughton supplied me with a 1:10” twist light Palma contour that I finished at 32” of length. I chose Broughton because their barrels have proven to be a step above others in the accuracy department. The Broughton crew really does an exceptional job of lapping and quality control and the results show on the target. I chambered the barrel with a ’95 Palma reamer and throated it out an additional .100” for longer seating depth. I intended this barrel to shoot the 210 grain Berger VLDs and it shoots them well. During this time period, I had become intrigued by some results my friend Burke Lott was getting with 155 grain bullets. I built Burke an F-Class gun this past spring. His barrel is a Broughton 1:10” twist that I chambered in .308 at his request. Burke tried the 155 grain Lapuas and his results were very good. At this time, I thought it was interesting that his 10-twist would shoot the 155s so well. After he brought the gun to the range about five weeks in a row and consistently shot at or under a half minute at 1000 yards, he really had my attention! I tried the Berger 155s in my 10-twist and they shot fantastically well. After considerable testing and shooting this combination at Camp Perry, I am confident that this is one of the very best shooting guns I’ve ever had.

 

[watercam]

I ended up with a 10" twist .308 Palma barrel due to a mistake on somebodies (nameless of course) part. Works great, never looked back again.

 

[Steve Blair]

I have a 10" twist .308 that shoots Berger Fullbores very precisely. My original intent was to shoot heavier bullets but magazine length made it inconvenient. After trying the Fullbores, there was no reason to look elsewhere.

 

[DaveTooley]

Good bullets and a good barrel are the major factors here. There is a balance between velocity and RPM's that has to be maintained. Exceed either and performance can be compromised

 

[clowdis]

Good bullets and a good barrel are the major factors here. There is a balance between velocity and RPM's that has to be maintained. Exceed either and performance can be compromised

I agree completely. What I have found over the years is that the faster twist barrels will shoot light weight bullets just as well as the slower twist barrels, but will probably like to do it at a slightly lower velocity.

 

[Turbulent Turtle]

I believe the main reason folks used to shoot with the slowest possible twist, was because of surprises that could be in bullets. If your bullet is spinning fast enough to be statically stable at the muzzle, it will only get more stable as velocity drops a lot faster than rate of spin does. If your bullet was not perfect, say a bubble in the core or a jacket issue, it could become dynamically unstable and at longer distance it could become a little unpredictable on the target.

However, in this day and age of superb bullets, and with people trimming an pointing meplats, the danger of a dynamic instability is very remote. Bryan Litz did write about how higher rates of spin can actually buy a few more BC points, I say "spin them fast," unless you're shooting pulled FMJ bullets. There is no reason a lighter bullet can't be shot in a 1:10 or faster twist. As long as the bullet does not self-immolate because it really is spinning too fast, the faster the better.

Once the bullet leaves the barrel, it has achieved its maximum velocity and its maximum spin rate, but the stability will keep increasing as the forward velocity diminishes.

 

[Ned Ludd]

I have always believed that under-spinning a bullet is far worse than over-spinning one. I have shot many the 40 gr Varmint bullet out of a 7-twist .224 barrel with no ill effects or apparent loss of precision. As other mentioned above, I have also shot plenty of 155s of a 10-twist barrel with no ill effects. In fact, I've shot plenty of 155s out of an 8-twist barrel, albeit one that is only 16" long and therefore has markedly reduced muzzle velocity relative to a 30" barrel. I'm sure there is an upper limit with regard to twist rate, muzzle velocity, and the onset of jacket failure. However, I think shooting 155s out of a 10-twist barrel is still a long way away from that limit.

 

[dcali]

It's a balance between wringing out the last percent of BC and minimizing the rifle's dispersion. At short range, slow twist is king. At some point, ballistics become more important and it's better to get the extra BC and suffer the increased dispersion, because dispersion is linear with range and wind is not.

 

[XTR]

It's nice to talk about the tech, but it wasn't the gun, esp shooting on a HP target with a 1MOA X ring, it is the trigger nut being properly aligned that makes the difference

 

[DaveTooley]

It's a balance between wringing out the last percent of BC and minimizing the rifle's dispersion. At short range, slow twist is king. At some point, ballistics become more important and it's better to get the extra BC and suffer the increased dispersion, because dispersion is linear with range and wind is not.

How do you get"extra BC"?

 

[XTR]

How do you get"extra BC"?

If I understand it correctly from what Mr. Litz has written there is a point where a bullet is marginally stabilized but the nose starts to yaw slightly. The effect is to lower the effective BC of the bullet, whereas a bullet that is fully stabilized doesn't yaw and maintains the full BC allowed by its form, so by making sure you fully stabilize your bullets you get the full effect of its form. Thus the idea of "extra BC". Hey, it's the best I can come up with

 

[Ned Ludd]

It's not really "extra BC". The BC of a given bullet is an inherent property of the bullet. However, as stated above, a certain minimum twist is required to minimize any pitch/yaw (and therefore drag) straight out of the muzzle and thereby obtain the maximum theoretical BC of any given bullet. Berger recently updated their recommended spin rates for their bullets to reflect a gyroscopic stability coefficient (Sg) of 1.50 as the minimum spin necessary to obtain maximal bullet BC under specific conditions. Spinning them any faster than that will increase aerodynamic jump. This may have a more adverse effect on short range precision than long range, where maximum theoretical BC means maximum resistance to external ballistic factors that reduce precision. The Berger twist rate calculator predicts that at 2950 fps, 59 degrees F, and 0 ft elevation, the 155.5 Fullbore bullet will require a minimal twist of 1:12.3 to achieve Sg = 1.50.

The fact that only a 12.3-twist is actually required under those conditions suggests to me that the manner in which the 10-twist barrel effects are described above could be a little misleading to some. Specifically, the 10-twist barrel may not have been an actual improvement over an 11-twist or 12-twist barrel, either of which would have provided the minimal 1.5 Sg. In contrast,the predicted Sg of a 13-twist barrel under the same conditions as above = 1.35, and a 14-twist barrel gives Sg = 1.17. Those calculations suggest the 13-twist and 14-twist barrels are underspinning the 155.5s, leading to an effective decrease in BC of 4% and 10%, respectively. That would be a noticeable drop in BC, and therefore performance, at 1000 yd. So it's not really that the 10-twist barrel is "improving" anything. Rather, it is simply [more than] sufficient to achieve the full theoretical inherent BC of the 155.5 gr bullet, whereas a 13- or 14-twist barrel is not.

On a somewhat different note, Bryan Litz has also shown that a higher spin rate can in some cases partially overcome dynamic instability issues such as are seen with the 168 SMK, thereby extending its effective range. Likewise, a faster twist may improve the trans-sonic stability of some bullets. I suspect that this type of increase in dynamic stability is not the major effect taking place in the scenario of a 155 gr bullet fired from a 10-twist barrel. Rather, it is simply that the twist rates of barrels that were commonly used for that weight bullet in the past (i.e. 13- or 14-twist) were simply insufficient to achieve maximal theoretical BC. A 10-twist barrel is sufficient...not necessarily better, but sufficient.

 

[dcali]

Put simply, a little more spin reduces the average yaw, which reduces yaw drag slightly. And since BC accounts for the *net* drag force as if the bullet were a point mass, you lose a little BC with barely stabilized bullets because they're pitching and yawing more relative to a more highly stable bullet. That's Bryan's finding at least, and it makes intuitive sense. It is a *small* effect, though, if you look at his numbers.

 

[Berger.Fan222]

I've gone as light as 125 grain bullets in a 1 in 10" twist and still had excellent accuracy with quality bullets. Never got good accuracy with 110 grain bullets in that twist though.

 

[Park ranger]

My blackout has a 1 in 8 twist and shoots 110’s very well. Of course I don’t try at great distance, but a case of extreme stability