Question on bullet stabilization, twist rates and velocity

[sig.natures]

I'm relatively new to the technical shooting world though a long time reloader of handgun and hunting rifle loads. I have a question about gauging appropriate velocity in varied twist rates for proper stabilization. I'm working with a Savage 12 Low Profile Varminter. I can get routine 1/2-3/4" group but wanting to move into the "precision" realm. I'm a bit confused. I know that a longer bullet in a given caliber requires a faster twist for stabilization. Here's my quandry. My rifle is a 22-250 with a 1:9 twist. Most reloading data for similar calibers is published with 1:12 to 1:14 twist. All of these publications tend to indicate that the loading most apt to produce the best groups is at the top of the velocity range.

Okay, so I look at a 22 PPC, a cartridge reknowned for its accuracy and see a recommended velocity with a 52 grain bullet around 3358 FPS in a 1:14 barrel. That equates to 3358/14*12=2878 revs per second. Now step up to a 22-250. Data says 3784 FPS at 1:14. That's 3243 RPS. A 22-250 load that corresponds with the 22 PPC's 2878 RPS is 2158 FPS in a 1:9 twist. Formula 2158/9*12=2877 RPS.

Still, the books say faster is better. What am I missing here? Either I shoot 22-250 loads as a much slower velocity in a 1:9 to equate to a recognized benchrest winner, or the 22 PPC would be more accurate at a faster speed than possible in that efficient casing.

Am I just over thinking this or do benchrest shooters really custom tune to the degree that I am discussing? Enlighten me!

Please send comments to [email protected]. I don't monitor this as often as I'd like but the other comes to me on my blackberry. Thanks.

 

[NCsmitty]

I prefer to answer here.
Consider yourself fortunate to have a 1in 9 twist in a great caliber like the 22-250. This gives you the ability to shoot much heavier bullets than 52gr. Normally, 22-250 is factory barreled with a 1in14, and that limits the bullet weights to 60gr or less.
By using a 69gr bullet for example, the velocity is automatically reduced due to the bullet weight and thereby, rotational speed. The heavier bullet also will give greater range and with proper selection of bullets, you can have a great long range rifle. Some match type bullets do have heavier jackets to thwart the higher rotational speeds.

NCsmitty

 

[sig.natures]

Thanks NCsmitty. I ordered the rifle with that twist. Savage offers both twists in the 12 LPV. The 65 grain bullets seem to offer the sweet stop at 22-250 velocities and I can get sub .5MOA pretty regularly. Best 3 shot group with 65 Sierra GAMEkings is 0.214" center to center. The Gamekings were all that I could find when I was looking for something in that range locally. I'll go to match bullets in that range next. the Nosler 69 HPBT does fairly well.

I'm really trying to understand the physics behind the calculations. I can understand why the thicker skinned match bullets hold together better against the centrifugal forces of higher rotational spin but is there a "sweet spot" of maximum stabilization for a given projectile shape and length? I have submitted the question to Sierra and all that I got back was a "the x bullet will stabilize in that twist."

Do you think that the formula example above equates the 53 grain 1:12 to 1:9 velocities is valid? Said differently, does the rotational speed achieved with a 22 BR at that velocity provide the same stabilization as the equivalent lower velocity in the faster twist?

 

[NCsmitty]

is there a "sweet spot" of maximum stabilization for a given projectile shape and length?

The simple answer is yes. But, there are lots of variables involved, as you might expect. You need to match the right velocity to that bullet from that barrel. The exact load from an identical rifle may give you different results. If you want match groups then all aspects need attention. From brass preparation through final COAL, it all matters.
Matching 22 BR rotational speeds is a thought but really not worth considering.
My only advice is to stick with what works. The Gamekings seem promising but you need to consider 5 shot groups to show their true consistency.
There are several true match type bullets that you can try. Berger and Lapua bullets are used regularly in competition with great results, if that's your goal .

NCsmitty

 

[rcmodel]

The secret to the 22 PPC, 22BR, and others is not a magic sweet spot with the rifling twist.

It is ideal case capacity for the bore, in conjunction with a shorter case, which makes a stiffer action possible.

The 22-250 can never match the accuracy of the bench-rest cartridges, no matter what you do.
It is over-bore capacity, and what you got is pretty darn good for it.

rc

 

[sig.natures]

over-bore capacity

rcmodel,

I appreciate that feedback. I'm trying to learn. My next investment will likely be a 6 PPC or BR. Help me with the term over-bore. I can understand how a less than case filling load could create unequal forces upon a bullet that extends past the neck into the chamber of the casing. I could invision that the expanding gas on the uphill, "empty" side might apply excessive pressure downward on the top of the projectile tending to angle it into the neck slightly. Is that a decent rendition? If so, what would the difference be if a bullet did not extend into the chamber of the casing? It would seem to me that the neck would act just as a pipe would with pressure building at the rate of powder burn offset by bullet travel that would in effect create additional "pipe" for expansion. Wouldn't that pressure be more or less even in terms of its impact on the bullet? Would the impact be less on a flat based bullet versus a boat tail?

 

[rcmodel]

Is that a decent rendition?

No.
Nothing at all like that happens. Fluid dynamics being what they are, equal pressure is applied to the base of the bullet as pressure builds, regardless of whether or not the case is full of powder. Whether it is a flat base or a boat-tail, it still gets equal pressure on the base of the bullet.

The problem with over-bore cartridges is discussed here much better then I can do it.
http://accurateshooter.wordpress.com/2008/05/05/overbore-cartridges-a-working-definition/

All I know is, it is much easier to get a round like the .308 or .222 Rem to shoot exceptionally well, then it is a 25-06 or .220 Swift.
For whatever reason.

That is not to say the 22-250 is not an accurate cartridge, Far from it.
But the BR calibers are more so.

rc

 

[Jim Watson]

Bear in mind that you are shooting a production line rifle. You may be getting all it has to offer. And what about the supporting cast of thousands? Do you have a high powered scope so you can see what you are doing? Do you have a good solid bench and stable rests? Do you use good bench technique? What is your brass prep procedure, what are your loading dies and set up how?

"Overbore" is kind of a nebulous term. RC takes overbore to mean a cartridge larger than what gives best accuracy. OK, but definitely empirical. 60 years ago, the .22-250 was IT in the game. But time has moved on.

The linked article is something else entirely. It computes a figure of merit that is supposed to relate to barrel life, not accuracy. Note that 6 PPC and 6x47 have exactly the same rating. This was probably not an accident by Pindel and Palmisono. But the PPC has completely displaced the 6x47, presumably because its chamber shape gives more consistent propulsion to the bullet.

The old definition of overbore was a cartridge too large for the powders available to give useful velocity increases over lesser rounds. That version of overbore is a moving target as "slower" more progressive powders become available.

As to bullet weight/length, testing trumps theory. The 9 twist .22 was introduced along with the 68 and 69 grain bullets in development of the AR as a match rifle. Worked pretty good... until they went to still heavier bullets in still faster twists.
But in the early days, Ed Harris did some shooting. He found that a 9 twist barrel, supposedly optimized for the 69 grain bullet, was still more accurate with the 52 grain bullet at 100 yards. The heavier bullet pays off when shooting at longer ranges in the wind.

Which relates to the 6 PPC and BR. Most of those are shot in 100-200 yard benchrest and have slow twists for light bullets. Savage is now offering their F class rifle in 6 BR with 8 twist for 107 grain bullets at 1000 yards, if that is your aim. If not, the LRPV 6 BR can be had with a 12 twist for 70 grain bullets.

 

[Scott23]

calculating twist rates

Divide the bullets length by its diameter=X. Then divide 150 by X and multiply the result by the caliber. Round to the next highest number and that will give the rate of twist for that particular bullet and caliber.

 

[AgentAdam]

As mentioned there are to many variables to say X twist will stabilize an X grain bullet the best. For one its the length of the bullet and not the weight that really dictates that. For instance the 63grn Penetrator/Ballistic Tip bullet is as long as a 70grner but im sure you already knew that. Another thing is a longer barrel will tend to stabilize any given bullet better than a shorter barrel of the same twist. For instance a 20+" 1/9 might like 70's were as some 16" or less don't. Also different barrels, even of the exact same specs, tend to like different bullet selections. Not only weight, but brand. Your best bet is to try every bullet,powder brand/charge,and COAL that you can.

 

[robcat]

bullet and twist rate

look it up on line and try it out; it Some time ago I bought a book called " High Power Rifle Accuracy" By Douglas E. Arnold, it contains a lot of good information. This book describes a mathematical equation called the Greenhill Formula, and it is used to determine the proper twist rate for a specific bullet caliber. I now use it to determine bullet purchases. I look like a fool at the store but who cares? I pull a bullet from the box and measures it length with a set of dial calipers. Than do the following...
GREENHILL FORMULA
1) Divide bullet length by bullet diameter. Example 1.04 divide by .257 cal.
= 4.046692607
2) 150 is a constant and is always used 150 divided 4.046692607
=37.06730769
3) multiply dividend from step2 by bullet diameter
37.06730769 x .257 =9.526298077 = 1in10 twist rate
look it up on line; it will increase bullet accuracy.

 

[mzimmers]

Hi, Robcat –

Thanks for introducing me to the Greenhill formula. I think this is going to help explain some of my frustration with using the larger copper bullets in my various rifles.

Note to all: Rob's rendition of the formula is fine for most applications, but needs a bit of tweaking in some instances:

1. for velocities above 2800 fps, the constant should be changed from 150 to 180.
2. if you're not using lead-core bullets, you need to adjust for this.

Details can be found at:

wiki on twist rates

Sigh...so much to know...