RIFLE TWIST

[Gunner3456]

I said "1:7 is OK for 62 and up for the target shooters."

I should have said "1:7 is BEST for up into the 70's for the target shooters, and will do OK for 62gr."

 

[coop44]

The biggest drawback I have with shooting 62 grain and heavier bullets is the freaking price. Even reloading the heavier bullets gets pricey. I'll stick with my obsolete, ineffective, unstablized, 55 grainers in my obsolete 1/9 twist barrel thank you.

 

[Otter]

Ideally you want to pick a bullet and twist combonation that perfectly matches. The twist should be right at what will stablize a given bullet weight/length. For instance if all you ever plan to do is shoot 55 grain bullets in a .223, don't go with a twist faster than you need, which is 1:12. A faster twist will always work (up to a point to where the bullet can actually come apart in flight), but there is no need to spin a bullet any faster than you need. Spinning faster than you need results in less accuracy. The cheaper quality the bullet the more impact this has. Bullets are not perfectly balanced, and a faster twist than needed exaggerates these imperfections resulting in less accuracy.

However, if you ever plan to shoot a heavier/longer bullet, go with the faster twist. A fast twist will always stablize a lighter bullet, however a slower twist cannot stablize a bullet after a certain weight/length is reached.

 

[Gunner3456]

Ideally you want to pick a bullet and twist combonation that perfectly matches. The twist should be right at what will stablize a given bullet weight/length. For instance if all you ever plan to do is shoot 55 grain bullets in a .223, don't go with a twist faster than you need, which is 1:12. A faster twist will always work (up to a point to where the bullet can actually come apart in flight), but there is no need to spin a bullet any faster than you need. Spinning faster than you need results in less accuracy. The cheaper quality the bullet the more impact this has. Bullets are not perfectly balanced, and a faster twist than needed exaggerates these imperfections resulting in less accuracy.

However, if you ever plan to shoot a heavier/longer bullet, go with the faster twist. A fast twist will always stablize a lighter bullet, however a slower twist cannot stablize a bullet after a certain weight/length is reached.

You are absolutely right. Nice post.

The military did have some rare issues with the 55gr breaking up in flight in the 1:7, so went to the 62gr for bullets with also the tracers.

Another issue is that 1:12 barrels are kind of specialty now. Off the shelf, affordable AR's are likely to have 1:9 or 1:7. I buy 1:9 so I can shoot 55 or 62gr, but I'll admit I have only 55gr now. My 1:9's shoot very nicely.

I think if the truth were known, the 55 is the better round. It's faster and flatter, and it really tends to tumble and break on impact, causing multiple wound channels. In 'Nam, while adjusting to the idea of such a small bullet, military members were really surprised at the damage they did. They became satisfied with them. I'm happy with it.

 

[xoddah]

Look up " green hill formula "" down load it is only a 718kb file
you can down load the calculator the most simple thing you ever saw the only info you have to input is

slower than 2800 fps or faster than 2800 fps check one
then enter bullet diameter
then enter bullet length

there is ...nothing else, the only thing it does is calculate twist rate

 

[chemist]

The purpose of twist is for stabilization; bullets are pointy and therefore are butt-heavy; a butt-heavy slug wants to fly butt-first; the longer the bullet, the more butt-heavy and therefore the faster the twist that's needed to stabilize it and keep the pointy end pointed downrange.

Accuracy gets worse with a faster twist, so the best twist is the minimum needed to stabilize the bullet.

The foregoing was all derived from .22LR experience, but the aero concepts are the same for any caliber or slug.

 

[Jamie6.5]

Actually chemist, Sierra's ballisticians have found the opposite to be true.
A bullet that is aft heavy is easier to stabilize, and a bullet that is nose heavy is harder to stabilize, requiring a faster twist.

I'd copy it for you, but it's in my Sierra reloading binder, not online.

 

[chemist]

But a pointy bullet is butt-heavy. Agreed?
And the heavy end wants to point forward when subjected to aero drag, right? Think shuttlecock.
All that's left is to decide is which way the center of mass moves as the slug gets longer. That would be rearward.

It's not rocket surgery.

A 40gr. .22LR slug stabilizes fine at 1:16", but a 60gr. requires 1:9" to keep it from keyholing. I'm not making any of this up myself; it's absolutely critical to prevent baffle strikes when shooting suppressed.

My 1:9" rifle that's dedicated to Aquila SSS 60gr. .22LR has inherently lower accuracy than a box-stock 10/22 shooting Federal HP - when using weighed and sorted ammo. It's kind of disappointing, but that's life - and physics.

 

[DieselScout]

Just fyi Shooting Times has an article discussing this in their issue this month. I haven't read it yet but their tech articles I've read to date have always been good.

 

[Sabertooth]

I now have a thundering head ache!!! But I get it.

 

[Jamie6.5]

Actually chemist, Sierra's ballisticians have found the opposite to be true.
A bullet that is aft heavy is easier to stabilize, and a bullet that is nose heavy is harder to stabilize, requiring a faster twist.

I'd copy it for you, but it's in my Sierra reloading binder, not online.

But a pointy bullet is butt-heavy. Agreed?
And the heavy end wants to point forward when subjected to aero drag, right? Think shuttlecock.
All that's left is to decide is which way the center of mass moves as the slug gets longer. That would be rearward.

It's not rocket surgery.

Actually, your statement above: "But a pointy bullet is butt-heavy. Agreed?
And the heavy end wants to point forward when subjected to aero drag, right?"
Doesn't ALWAYS hold true, and the way you state it, sounds backwards IMO.
And is not necessarily so. As Sierra states, the actual data is counterintuitive.

Sierra explains that they tried hollow boat tails, and boat tails made entirely of jacket material, thinking they'd stabilize more easily with the CoM more forward, and found exactly the opposite.

I'm not trying to make the case. I'm just quoting Sierra's people, that get paid for the application of their knowledge.
If you want to argue, write them a letter. But before you do, you really should read their dissertation.

It truth, the determining factor in finding the ideal twist is bullet length, regardless of weight.
We falsely attribute it to weight, because we equate one with the other, when that is not always the case. Especially when different materials are factored in.

A 1.5" long monometal copper bullet won't weigh near as much as a jacketed lead version of the same 1.5" length, but will require the same twist rate to stabilize. Regardless of ogive length etc.

 

[chemist]

This is indeed getting interesting. All I know comes from rimfirecentral.com, and there's no comparable boat-tail slug geometry in .22LR.

Aero is a whole separate topic from mass distribution, and I suppose that variances in the ogive shape could swamp the COM-effect. So I'm guessing that a BTHP could have an entirely different set of stabilization constraints from a more conventionally shaped slug.

I think your point about a Cu slug tells the tale: even though it weighs less, its COM is in the same place as the lead-slug COM, ergo it requires the same minimum twist rate to stabilize.

 

[Jamie6.5]

I've wondered. Does it matter if it twists to the right or to the left? Do all manufacturers twist in the same direction?

Historically, no and no.
Currently prevailing "wisdom" seems to be right-hand (clockwise when viewed from the breech) but both have been used/tried.

 

[chemist]

Now that's a question! Why don't the Ozzies spin their slugs t'other way?

 

[skydiver]

Now that's a question! Why don't the Ozzies spin their slugs t'other way?

Think about it...toilets flush in the opposite direction below the equator, so why shouldn't bullets spin better in that direction as well.

 

[old_heli_logger]

Great thread! Great info...I knew if this went long enough, we'd eventually end up in the toilet