I have a question about twist rates!

I have a question about twist rates! - Printable Version

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I have a question about twist rates! - Mark611 - 11-01-2023

for example, is a 1&8 a true 1&8 or is it a 1.75 or a 1.785? what say you? Cool

I have a question about twist rates! - Klem - 11-01-2023

Mark,

If it is engraved 1:8 it will be exactly that.

The slightly faster twists help stabilize heavier bullets in bigger calibres like the Creedmore. You don't need it in Grendel however because the smaller case favors lighter bullets. There are also potential downsides to faster twists so it's not just faster is better.

If you want to check, put an oversized patch on your cleaning rod. Start it down the barrel and it rotates with the rifling (as it should). Note the distance it takes to do one full rotation and measure.

I have a question about twist rates! - Mark611 - 11-02-2023

Thanks, Klem, I just wondered if there was any deviation to those numbers listed on twist rates,

I have a question about twist rates! - Legionnaire - 11-02-2023

Not in my experience. I've measured multiple barrels the way Klem suggests. Sometimes I make multiple passes to confirm.

I have a question about twist rates! - SDW - 11-02-2023

Not really related to the OP's question, but I remember reading somewhere that if you have a rifle with a short barrel you'd want the twist to be slightly faster than if it had a longer barrel, all other things, including ammo, being equal. Supposedly it relates to MV and spin rate. Bullet doesn't come out as fast, so a faster twist will increase its 'RPMs' so to speak and help with stability (to be on par how it'd be with a longer barrel).

Sounded good to me at the time. I have no idea if there's any truth to it. Or if it even matters all that much in the general scheme.

I have a question about twist rates! - LRRPF52 - 11-02-2023

When Bill did the initial and continuing testing with 6.5 Grendel, which included many trips out to New Mexico for high sand and dust, plus long range, he found that the 20 and longer did well with 1/9 twist.

For 18 and less, he preferred 1/7.5 twist.

You can still get by with 1/8 twist in 18 barrels shooting to a certain distance which is primarily determined by your bullet choice, mv, and density altitude.

Lower elevation with thicker air conditions are best countered with faster twist if you want gyroscopic stability to help push into transonic mach range.

I have had better-than-expected results at 1000yds with 1/8 twist 18 Lilja barrels at 6300ft elevation.

I need to shoot some more 4400ft elevation with 1/8 twist to see if Im coming apart there.

With my original 16 AA barrel/bolt combo build (1/7.5 twist), I had surprisingly-predictable impacts at 1200yds with 123gr A-MAX at 4400ft elevation in the summer.

I have a question about twist rates! - Klem - 11-02-2023

I will add that you don't want to overstabilize bullets by having too fast a twist - they resist pointing down to the target after apogee. This is more noticeable in long range shooting.

Here is Shilen Rifles advice on twist rate:
[INDENT]6.5mm/.264
- 7" * for bullets heavier than 130 gr.
- 8" for bullets heavier than 130 gr.
- 8" * Ratchet rifled 4 groove
- 9" for bullets up to 130 gr.[/INDENT]

Kreiger twist rates for 6.5 are 1:7, 1:7.5, 1:8, 1:8.5, 1:9.

1:8 is the standard as it sits in the middle of what works. Still, lighter 6.5 bullets are more suited to 1:8 and slower.

I had a 1:7.5 to shoot 140gn Noslers in 6.5x47. Turned out it was less accurate than the Kriegers. I suspect that was down to the quality of the Pac-Nor barrel and not the twist rate, but I'll never know for sure.

I have a question about twist rates! - SDW - 11-03-2023

Gonna say, a bore brush works fine also for doing the measurement, whatever is handy. It was interesting to do this on (in?) my 1894 Marlin 44mag one time. 20" barrel and the rod just made it about half way through one spin before the brush came out the muzzle. LOL The rifle's got a 1:38" twist for some reason. I don't know why Marlin made them that way. It shoots okay for the kind of rifle it is, but it's certainly no tack driver.

I have a question about twist rates! - kmon - 11-05-2023

Berger twist rate calculator is pretty good at determining what you will need. https://bergerbullets.com/twist-rate-calculator/
There is a list of the berger bullets to select from but know a few things about you bullet and input those then calculate.

Polymer tipped bullets do not count the tip in the length for calculation.

SDW the 1:38 should be fine for a 44mag with 180 or 200gr bullets,I have seen some shoot 240s ok but others not so much.

I have a question about twist rates! - Konocti's Wigwam - 11-13-2023

Klem Wrote:I will add that you don't want to overstabilize bullets by having too fast a twist

I don't think it matters too much, it's not like 1:5 used on 300 blackout, or 1:3 used on 8.6 blackout. The twist has more to do with the projectile you use.

I just received a 12" Faxon 6.5 Grendel barrel with 1:8 twist. I plan to shoot 100-120 grain projectiles. If I wanted to shoot heavier bullets I would look for a 1:7.5 or 1:7 twist barrel. I don't see a reason to do so, however. If I need heavier bullets I'll use 8.6 blackout.

KW

I have a question about twist rates! - lazyengineer - 11-14-2023

Klem Wrote:Mark,

If it is engraved 1:8 it will be exactly that.

The slightly faster twists help stabilize heavier bullets in bigger calibres like the Creedmore. You don't need it in Grendel however because the smaller case favors lighter bullets. There are also potential downsides to faster twists so it's not just faster is better.

If you want to check, put an oversized patch on your cleaning rod. Start it down the barrel and it rotates with the rifling (as it should). Note the distance it takes to do one full rotation and measure.

Does that actually work? Not being sparky, just wondering if the experimental error of that is enough to tell 1:8 vs 1:8.2,since everyone always says there are manufacturing aspects and its super common for a twist rate to be slower than specified (which I have no idea why that would be true ...)

I have a question about twist rates! - Klem - 11-14-2023

Konocti's Wigwam Wrote:I don't think it matters too much, it's not like 1:5 used on 300 blackout, or 1:3 used on 8.6 blackout. The twist has more to do with the projectile you use.

I just received a 12" Faxon 6.5 Grendel barrel with 1:8 twist. I plan to shoot 100-120 grain projectiles. If I wanted to shoot heavier bullets I would look for a 1:7.5 or 1:7 twist barrel. I don't see a reason to do so, however. If I need heavier bullets I'll use 8.6 blackout.

KW

Mate, I wouldn't be so dismissive of RPM. I have seen a suppressor smashed due to a bullet coming apart inside (LeHigh/Blackout). Also seen puffs of smoke prior to the target and then no hole in the target at all. Depending on the velocity, bullet, and twist, RPM does all sorts of things and we don't pay enough attention to - including me.

Never been happy shooting animals with heavy subsonics; it's like shooting them with an accurate handgun, they rarely die immediately (humanely).

Here's a neat summary on Accurate Shooter.
https://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

I have a question about twist rates! - Klem - 11-14-2023

lazyengineer Wrote:Does that actually work? Not being sparky, just wondering if the experimental error of that is enough to tell 1:8 vs 1:8.2,since everyone always says there are manufacturing aspects and its super common for a twist rate to be slower than specified (which I have no idea why that would be true ...)

On the few occasions I've done it, it works. Watch as is slowly moves in a circle and mark the points with a black texta.

As for seeing the difference between 8 and 8.2 I don't know, but a jammed tight patch does not slip so I don't see why not.

I have a question about twist rates! - Konocti's Wigwam - 11-14-2023

Klem Wrote:Mate, I wouldn't be so dismissive of RPM.

I've heard of that also, but not with Grendel. Grendel seems to be one of the more forgiving rounds. With 8.6 blackout using a 1:3 twist, the projectile will spin at over 500,000 rpm.

I have a question about twist rates! - LRRPF52 - 11-14-2023

lazyengineer Wrote:Does that actually work? Not being sparky, just wondering if the experimental error of that is enough to tell 1:8 vs 1:8.2,since everyone always says there are manufacturing aspects and its super common for a twist rate to be slower than specified (which I have no idea why that would be true ...)

Yes, this is how I used to measure my twist rate with a rod and piece of tape on it with an indicator/witness mark.

You can see where it makes a full rotation, then mark that length on the rod. Once the rod is removed from the barrel, measure how far that point is that it took to make a full revolution, and there is your twist rate.

I have a question about twist rates! - SDW - 11-18-2023

kmon Wrote:SDW the 1:38 should be fine for a 44mag with 180 or 200gr bullets, I have seen some shoot 240s ok but others not so much.

Oh, I'm familiar with what works. I've owned the rifle over 20 years now. It shoots some 240s great. No need to go into detail on that here other than to say I need to use bullets at least .430" or a little larger due to the SAAMI "rifle" spec bore. That's proven to be more important than bullet weight, if we're talking group size.

Back to Grendels, BFT specs his SixFive barrels to use 1:7.5. At least the under-20" barrels.