1:75 vs 1:8 - anybody tell the difference and if so , where?

[bob4432]

Title says it all.

Please explain if there is a difference.
Thanks,
Bob

[Klem]

Title says it all.

Please explain if there is a difference.
Thanks,
Bob

Bob,

Depending on the quality of gun, ammunition, rest, weather conditions, range, and your ability to shoot you probably won't notice the difference. Or more importantly, not be able to tell whether the twist is the reason for the group size.

6.5's come in a range from 1:7 to 1:9 with the majority of drop-ins in the middle 1:8. I have shot a 1:7.5 Pac Nor 'Super Match' in 6.5x47 shooting 130gn bullets and got larger groups than 1:8 Kriegers, but am tempted to think it's the barrel quality rather than the twist. I've only had one faster than normal twist barrel in 6.5mm so we should not be generalizing to your situation. In 223 F Class I have used a slightly faster twist 1:7", than the normal 1:8", to stabilize the heaviest 90gn Berger bullets, but again results on target were a big yawn.

In 6.5mm Shilen recommends 1:7"-1:8" for bullets 130gn and heavier, and for lighter than 130gn, 1:8"-1:9".

In 300Blackout, in the early days the standard was 1:8" but is now 1:7" across the industry, to stabilize the heaviest 200-220gn subsonics.

The point being, faster twists to stabilize heavier bullets and slower twists if exclusively shooting lighter bullets is the general principle, but there can be a cost to spinning bullets too fast. Over-stabilized rounds resist pointing to the target after apogee, so for long range shots this means wider groups. Plus any imperfections in the bullet get magnified the faster you spin it, again larger groups. In extreme cases overspinning a bullet has it coming apart somewhere downrange before the target. LeHigh now includes advice on spin RPM for their 30cal Blackout type bullets after they were coming apart in shooters' suppressors, damaging them.

To be fair, the difference you are considering is slight and if anything favors heavier bullets. Up to you but I doubt you'll notice any difference (all other things being equal).

[Zeneffect]

In a grendel couldn't tell you. In .223 I can tell you with a 80gr bullet or heavier I prefer faster than a 1:8 twist as I have experienced destabilization when pushing the projectile past 3000fps. This has lead me to a thought of math vs reality.

My experience shows a difference, yet I shoot with guys who are successful. The only difference is the barrel manufacturer, so there is where the question arises. Am I REALLY 1:8 or is it slower.... or is theirs really 1:8 and is really faster?

I came up with 2 solutions and ran them both. I switched barrels to a krieger 1:7.7 and issue went away.

I slowed down to 2900fps and issue went away.

Comparing the results of the 2 contrasting by effort and cost, simply adjusting my load was a far more efficient solution. The extra 100fps gained me nearly nothing on target, well within error of shooter.

What did I learn? If I'm asking the question of 1:7 vs 1:8 just stop. Spend more time in load development, going the other route will put an unnecessary dent in your wallet that you would be far happier with putting towards an optic.

[bob4432]

Appreciate the knowledge gentlemen. I have my own thoughts on the matter and know I am not in the "top of the shooting class" so I came here to see if those with more shooting time had an overwhelming yes or no.

[BluntForceTrauma]

My opinion on this issue has evolved.

I set twist rates for the worst-case scenario, which I define as a 12" Grendel on the Alaskan coast in winter. So I go to the Berger Twist Rate Stability Calculator and select from the drop-down menu a given 6.5mm bullet, then enter 2000 fps for muzzle velocity and 0° F temperature and 0 feet altitude. Then I start playing with twist rates.

In prior years, my assumption was that I should specify barrels for that rare shooter who might want to use a 140-grain bullet in the Grendel so I specified a 1:7.5 twist. Now, however, I don't think a 140gr is realistic in the Grendel, and that 130gr is the maximum realistic weight. So using Berger's calculator with my worst-case scenario and selecting their 130gr Hybrid OTM, I find a 1:8 twist gives full stability to my SBR. (Not that I'd really be worried about losing 1% or 2% ballistic coefficient from an SBR!)

I used to think there was no downside to over-stabilizing a bullet (talking 6.5 Grendel here, not about pills coming apart at 4,000 fps), because a Brian Litz study said it could help accuracy at long range and the fps loss from tighter twist rates was ridiculously low — about 1.25 fps per twist inch.

However, there is also the consideration that microscopic imperfections in bullet manufacturing — the microscopic voids in lead cores or the non-concentricity of both jacketed and monolithic turned bullets — can cause a bullet to slowly wobble off target due to uneven centrifugal forces. These imperfections are exacerbated by higher twist rates.

So, on the one hand we want tighter twist rates for accuracy. On the other hand, we want looser twist rates also for accuracy. What to do?

Set the Goldilocks "just-right" twist rate. For mass-produced 6.5 Grendel barrels shooting a max 130gr bullet weight, my opinion based on the reasoning above is that it's 1:8.

[Zeneffect]

And that .25 inch gain at 300 yards will be negated by a squirrel farting in the woods 30 feet away from the target. Lots of stuff we obsess over don't really matter in practical application.

[BluntForceTrauma]

ZE, agreed. But one has to, in the end, spec one twist rate or the other. So might as well spec it rationally and let the farts fly where they may....

[JASmith]

Posts 5 & 7 -- Well said !!

[LRRPF52]

When Bill Alexander did the initial testing with different barrel lengths for 6.5 Grendel, he found that 1/8.75” and 1/9” stabilized the LR bullets fine from 20” and longer barrels.

For 18” and shorter, he went with 1/7.5” twist.

I recall that my first 16” AA barrel was the tight 1/7.5 twist, and was very predictable at 1200yds with 123gr A-MAX.

My 17.6” Lilja with 1/8 twist is predictable out to 1000yds at 6600ft elevation/23.4” hg, but I’m not sure what it does after that. I’m trying to recall if I’ve stretched it out past 1000yds and am drawing a blank.

[kmon]

My Grendel Boltaction has a 1:8.4 22 inch and does fine out to 600 yards with 130gr Berger. I usually go with faster twist but when offered a good deal on an Obermeyer barrel why not take it.

I have a couple that I wish were a little faster twist for heavier bullets but to me I can run the 90gr bullets in the 1:8 22-243 27 inch barrel but really have to stand on the gas to get acceptable accuracy.

In selecting between those for the Grendel I would go 7.5 twist especially if going with a shorter barrel.

Berger twist rate calculator is fun to play with and can be informative

[Bonas]

This is a concise article from Lapua discussing stability, twist, and their stability calculator: https://www.lapua.com/lapua-ballistics-t...stimation/

It's also a good reminder that twist rate and stability relate to a bullet's length, not its weight. Oftentimes, weight is a good enough indicator of length but not always.

[lazyengineer]

My opinion on this issue has evolved.

I set twist rates for the worst-case scenario, which I define as a 12" Grendel on the Alaskan coast in winter. So I go to the Berger Twist Rate Stability Calculator and select from the drop-down menu a given 6.5mm bullet, then enter 2000 fps for muzzle velocity and 0° F temperature and 0 feet altitude. Then I start playing with twist rates.

In prior years, my assumption was that I should specify barrels for that rare shooter who might want to use a 140-grain bullet in the Grendel so I specified a 1:7.5 twist. Now, however, I don't think a 140gr is realistic in the Grendel, and that 130gr is the maximum realistic weight. So using Berger's calculator with my worst-case scenario and selecting their 130gr Hybrid OTM, I find a 1:8 twist gives full stability to my SBR. (Not that I'd really be worried about losing 1% or 2% ballistic coefficient from an SBR!)

I used to think there was no downside to over-stabilizing a bullet (talking 6.5 Grendel here, not about pills coming apart at 4,000 fps), because a Brian Litz study said it could help accuracy at long range and the fps loss from tighter twist rates was ridiculously low — about 1.25 fps per twist inch.

However, there is also the consideration that microscopic imperfections in bullet manufacturing — the microscopic voids in lead cores or the non-concentricity of both jacketed and monolithic turned bullets — can cause a bullet to slowly wobble off target due to uneven centrifugal forces. These imperfections are exacerbated by higher twist rates.

So, on the one hand we want tighter twist rates for accuracy. On the other hand, we want looser twist rates also for accuracy. What to do?

Set the Goldilocks "just-right" twist rate. For mass-produced 6.5 Grendel barrels shooting a max 130gr bullet weight, my opinion based on the reasoning above is that it's 1:8.

The last part in particular. Personally I run 1 / 9 in 20", and run 1/8 in everything shorter - and have been happy. I have little desire to run 1/ 7.5, but I suppose if you are planning on running 140's, in a 12", it would matter.