Three choices

[Lucky6547]

Hello,
Goal: 600yard ftr
Bullet: 80gr bergers
Three barrels to choose from:
30" 8twist
28" 7twist
26" 7.7twist
Thoughts?
Thanks,
Ken

 

[Cold Bore]

Hello,
Goal: 600yard ftr
Bullet: 80gr bergers
Three barrels to choose from:
30" 8twist
28" 7twist
26" 7.7twist
Thoughts?
Thanks,
Ken

There's my vote.

I'll take the insurance of the faster twist when shooting 80's (while also giving you the ability to move up in weight later if you decide to try it), and give up the two inches of barrel.

The shorter mid-twist barrel loses on both accounts, so isn't a contender for me.

 

[TAJ45]

There's my vote.

I'll take the insurance of the faster twist when shooting 80's (while also giving you the ability to move up in weight later if you decide to try it), and give up the two inches of barrel.

The shorter mid-twist barrel loses on both accounts, so isn't a contender for me.

++ Two - both counts. What itty bitty bit of velocity gained by the .7" less twist is more than compensated for by the extra 2" of bbl, not to mention set back length should you get crazy and run 5-10k rds thru it.

 

[1lnbrdg]

For .223, all of my barrels will say 7/something. If you want to try the 90gr bullets later, the 1/7 will do those. Beyond that, either of the 7-twists will work.

 

[Ned Ludd]

Based solely on the limited info you provided for those barrels, I'd also vote for the 28" 7-twist. However, if I was making the purchase I'd also want to know the contour and land/groove pattern. The alternative I'd go with would be a Krieger 31" .218/.224 SS 4-groove Heavy Palma in 7-twist. Twist rate has little to do with velocity; it's all about what bullet you can effectively stabilize. With a 30" finish length, you wouldn't be restricted in terms of length (i.e. velocity) or twist rate for any bullet you might want to shoot. As long as you can make weight with a 30" pipe, all you'd need to do is to have it chambered appropriately. Southern Precision currently has them in stock:

http://bugholes.com/index.php?route=product/product&path=88_89_96&product_id=551

 

[r bose]

Bergers website now recommends 7 Twist for anything heavier than a 75g VLD. The 80, 80.5, and 82g all say 7 TW next to them. That's my vote...and yes the longer the better ^^^^^^

 

[TAJ45]

Twist rate has little to do with velocity; [/QUOTE]

I'll have to disagree here, to a point. Little to do, yes, however it does effect velocity. The BR boys want their PPCs to stabilize whatever they're shooting but no more. Shoving a mass through a hole with the mass obdurated, requiring that mass to rotate, requires energy to do that work. The more twist, the more work needs to be done = more energy required. Therefore less available for speed = more flight time = more wind susceptibility.

As I stated , small gain in fps w the 7.7 over the 7tw.

Can't remember his real name but James Calhoun, according to a close enough customer would not do an 11 tw on his 19 Calhoun's w the PN bbls as he was very, very velocity driven .

 

[Ned Ludd]

The effect of twist rate on velocity has been extensively tested. Here is one example:

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

Within the ranges typically used within a given caliber, the effect is negligible. That is to say, it falls well within the ES/SD values observed and can't be reliably measured with typical chronographs. The anecdotal way it is typically measured (i.e. - someone has a certain twist barrel that in their hands is faster/slower than another of the same length with a different twist rate) is much more likely be some inherent difference between the barrels other than twist rate. Some barrels are faster/slower than others of the same length, even with the same twist rate.

In Bryan's example at the link above, the difference in average MV between 8-twist and 12-twist .308 barrels was 8 fps. That is less than the 12 fps and 11 fps SD values measured for the same barrels, respectively. In other words, the effect of changing twist rate from 1 in 8" to 1 in 12" on velocity was within the standard deviation of the samples. As I stated before, most people using typical chronographs would never be able to reliably measure a difference in velocity due to twist rate when going from a 7.7 twist to a 7.0 twist.

 

[Lucky6547]

Very interesting points...thanks for the info guys!
Ken

 

[TAJ45]

Very interesting points...thanks for the info guys!
Ken

I too thank you GS--- for your well presented, dispassionate information. I truly enjoy, actually relish, learning.

Not being the least bit knowledgeable in the realm of statistics and interpretation it is just a bit of a conundrum for me, knowing that "identical" bbls can differ so much in fps of how that can be reconciled in my pea brain.
I have a very anal friend who did tests on chambering to tighter and tighter run out tolerances to determine effects on accuracy.
An accomplished BR competitor, his findings were quite interesting.
I mulled the stat thing during breakfast and am guessing if the sampling was large enough, the results would be meaningful.

Again, thank you for the intelligent discourse.

 

[Sniper338]

28 inch 7

 

[Ned Ludd]

I too thank you GS--- for your well presented, dispassionate information. I truly enjoy, actually relish, learning.

Not being the least bit knowledgeable in the realm of statistics and interpretation it is just a bit of a conundrum for me, knowing that "identical" bbls can differ so much in fps of how that can be reconciled in my pea brain.
I have a very anal friend who did tests on chambering to tighter and tighter run out tolerances to determine effects on accuracy.
An accomplished BR competitor, his findings were quite interesting.
I mulled the stat thing during breakfast and am guessing if the sampling was large enough, the results would be meaningful.

Again, thank you for the intelligent discourse.

The simplest explanation of the statistics is that if the sampling error rates of different group measurements (in this case the SD values) are larger than the actual difference between the average measurements for each group (in this case the velocities), then the values cannot be considered statistically different. Basically, it means that two averaged values can't be considered numerically different (even if the numbers are not the same) if the measurement error/variance of each group is larger than the difference between them. The difference gets "lost in the noise" so to speak.

There are several problems with making these kinds of measurements in a meaningful way. Bryan's tests did about as good a job as can be done to isolate the effect of twist rate on velocity. However, it's been well documented that different barrels, even those of identical length, contour, chambering, and twist rate, can produce different velocities with the exact same load. It is not clear to me what the underlying causes of that are, but it's not uncommon to see differences of 20 fps, 30 fps, or more, even in barrels that otherwise appear to be "identical". It just happens. From a purely theoretical viewpoint, you're absolutely correct that a faster twist should cause a decrease in velocity. But from a practical perspective, if the decrease in velocity is very close to (or below) the minimum velocity that the typical chronograph can read, or is less than the variance obtained within a velocity group measurement, we can ignore it as statistically irrelevant.

I wasn't attempting to belittle you or anyone else in the above post, I was simply trying to point out that the effect of twist rate on velocity shouldn't be a major consideration in the barrel selection issue outlined by the OP. It's far more important to make sure the twist rate is sufficient to fully stabilize whatever bullets will be used, that the chambering is optimized for loading those bullets, and that the contour is appropriate for the intended use. I've been very, very pleased with the precision I get from several different .223 and .308 F-TR rifles I have, but at very close to 18 lbs, I'd never consider carrying one through the woods or up a mountain on a hunting trip .