Twist and shout

[AK5.56]

I'm dipping into the world of bolt action, semi long distance and hunting. I jailed a R700 AAC-SD (.308) and get to pick it up on Friday . When filling out the DROS and chatting with the sales guy he made a statement that this rifle is better than the R700 sps tactical (what I originally wanted but they were out so I got the AAC) because of the 1/10 twist ratio. That got me thinking

Now I've done some research and I have a basic understanding for twist ratio. I understand that a faster twist stabilizes a heavier bullet but I still have a couple questions.

First off is why? I would get how twisting the bullet at different rates would cause stabilization if the bullet had fins on it but its smooth so I don't get how twisting it at different rates affects the stabilization and how that relates to the weight of the bullet.

Second, would a faster twist rate hinder the performance of a lighter bullet? So in my case between the sps tactical with a 1/12 ratio and the AAC-SD with a 1/10 ration will shooting 150grn in the aac-sd perform less adequately than the sps tactical?

Third, When would you prefer a faster twist ratio over a slower one and vice versa for certain applications? Im having trouble figuring out how you apply the ratios to the type of shooting you are doing, bullet weight aside.

Any feedback greatly appreciated as always from the calguns society!

-AK

[CarlB]

I found it all to be about gyroscopic stability (GC) which is dictated by bullet weight which pertains length, ballistic coefficient and well as muzzle velocity.
Figure what bullet you want to run and at what muzzle velocity and you will know the twist rate you want in the barrel. Bryan Litz wrote a good book called Applied Ballistics for Long Range Shooting that goes into exact detail about this.

[jpx0123]

I'm new to this as well. But from my understanding the faster twist rates are better for stab aliasing heavier grains also seen this to be true for ar barrels as well. The 1:12 works with 140-180 grain while the 1:10 works with 150-220 grain. I think both would work fine with the 168 gr is what I've seen people saying. As for actual experience, I'm working on that myself. As to why this is true a little to technical for my understanding at this time but I'm sure the more experienced members will chime in on that.

[AK5.56]

That seems very limiting. Does that mean twist rate will ultimately affect accuracy with different bullet weights? Im not going to change the barrel for the multiple different bullet weights I will be shooting. Im just curious as to the affects of the twist rate in various situations in basic terms.

[jpx0123]

I believe it does affect accuracy. But whichever twist you choose you would still encounter this issue. It's a matter of what range of weight you want to be in. But I've also been told 150gr rounds which are available everywhere and at the lower end of the cost spectrum will shoot fine. But if you are going for sub moa groups you want to look into the heavier weight ammo

[AK5.56]

So a faster twist with heavier bullets would be considered desirable for accuracy over a slower twist and lighter bullets? Would the range of shot change this?

[jpx0123]

I'm pretty new to this world too. But distance does come into play I think. I went with the 1:10 twist myself. But am planning on working on my accuracy in increments. Start at 100 and work my way up as far as I can go.

[as_rocketman]

It depends on what you're doing.

If you're field shooting at close to medium range, lighter bullets are preferred -- they go faster for a given powder charge, they have less recoil, and they have a flatter trajectory that requires less adjustment (or no adjustment) for distance.

And they're cheaper. Lighter bullets don't need as much twist. At the extreme end, truly high velocity light bullets can actually fly apart with too much twist, though you tend to see this with barrel-burner cartridges like .220 Swift.

On the other hand, if you're precision shooting, or you're reaching out beyond medium range, heavier bullets are preferred, and that mandates a higher twist rate. The heavier bullet will initially be slower and have a steeper trajectory, but it retains more velocity downrange, so it eventually has a flatter curve. It will be less affected by wind and will travel farther before going sonic. In almost all cases, going to a heavier bullet also means enabling bullets with much, much better ballistic coefficients.

.308 Win is one of the great flexible calibers. If you're going to shoot basic 150 gr FMJ at medium range, you are unlikely to see any practical difference between 1:12" and 1:10" twist. However, if you go to longer range, you'll do better with 168 gr or 175 gr ammo (etc.), and for that the 1:10" barrel may perform better.

[ExtremeX]

That seems very limiting. Does that mean twist rate will ultimately affect accuracy with different bullet weights? Im not going to change the barrel for the multiple different bullet weights I will be shooting. Im just curious as to the affects of the twist rate in various situations in basic terms.

I believe it does affect accuracy. But whichever twist you choose you would still encounter this issue. It's a matter of what range of weight you want to be in. But I've also been told 150gr rounds which are available everywhere and at the lower end of the cost spectrum will shoot fine. But if you are going for sub moa groups you want to look into the heavier weight ammo

It doesn't affect accuracy... it affects gyroscopic stability which MAY affect accuracy.

There is more to gyroscopic stability than just bullet weight and twist rate. LENGTH of the projectile and the VELOCITY in which it’s being shot at also plays into the stability calculations.

For example if 200gr bullet is too “heavy” for a 1:12 twist rate and its causing accuracy issues, theoretically if you can shoot it fast enough, it would stabilize.

Here is a link to a calculator so you can play with the figures and see for yourself.

http://www.bergerbullets.com/litz/TwistRuleAlt.php

When reloading, powder selection can be a make or break a load when dealing with projectiles which are on the borderline of gyroscopic stability as you may be able to achieve higher velocity with different powders.

[ExtremeX]

So a faster twist with heavier bullets would be considered desirable for accuracy over a slower twist and lighter bullets? Would the range of shot change this?

If the bullet is stable in flight, it’s stable in flight. If you are running off a good accuracy node for that rifle and load, then heavy or lighter bullet makes no difference.

At range you need to worry more about when the projectile goes transonic. When the bullet goes thought the transition is when ugly things start to happen.

This is why bullets with higher BC values are sought after as they are more aerodynamic and retain its velocity in supersonic flight longer than bullets with lesser BC values and higher drag. More drag = faster decay in velocity even though it may have exited the muzzle faster than the heavier bullet with a higher BC.

[CobraRed]

+1
Stability is about length. But since the caliber/diameter is always going to be .30 in this case, heavier bullets are longer.

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[NorCalFocus]

To make it really simple, most target loads for a Rem 308 use the Match King 168 or 175. They both seem to work well in either the 10 or 12 twist. That's just off what I've read and seen though.

[1859sharps]

my thoughts for what they are worth......

1. it's too late to worry...you already bought the rifle
2. see #1

it has always been my understanding it isn't so much the weight of the bullet, rather than length of the bullet that determines twist.

it just happens that heavier tends to mean longer, lighter tends to mean shorter.

Anyway, when you say "lighter" are you talking less than 150 grain? if not, I wouldn't worry about having the faster twist and using "lighter" bullets.

[Xtracrispy]

my 1-10 SPS AAC seems to shoot just as well with 155's as it does with 190's, of course this is is my relatively unskilled hands, which are not able to put the rifle to its full ability with either load, still easily sub-moa though.

Just stop worrying and enjoy it

[AK5.56]

Thanks for the awesome feed back gentlemen! I find this topic very interesting.

theoretically if you can shoot it fast enough, it would stabilize.

The way I'm understanding this is how a top(child's toy) works. The heavier and faster it is initially spun the longer it spins in one spot with out wobbling.

Transonic would be when the top loses its momentum and starts wobbling on the table

To make it really simple, most target loads for a Rem 308 use the Match King 168 or 175. They both seem to work well in either the 10 or 12 twist. That's just off what I've read and seen though.

I was more interested on how I would be affected shooting 150 grn as they are cheaper and easier to find.

my thoughts for what they are worth......

1. it's too late to worry...you already bought the rifle
2. see #1

it has always been my understanding it isn't so much the weight of the bullet, rather than length of the bullet that determines twist.

it just happens that heavier tends to mean longer, lighter tends to mean shorter.

Anyway, when you say "lighter" are you talking less than 150 grain? if not, I wouldn't worry about having the faster twist and using "lighter" bullets.

Wasn't worrying, just curious. Didn't even doubt my purchase just wanted to learn more about the hobby that's draining my wallet.

By length do you mean actual bullet length out of the case or the total length of the cartridge. Would this be affected to how far the bullet is pushed into the case when reloading?