Prefered Bullet Weight For A Given Barrel Twist

rrogacki

Forget all the other aspects of creating a load, has anyone found that a particular weight bullet will always shoot more accurately given a particulat rate of twist for a given caliber. An example would be a 95 grain bullet for a rate of twist of 1 turn in 10 for a 243. I am curious, there have been a few reloading myths busted here over the last few weeks and would really like to see what some of the senior (and I don't mean old) members think.

Deleted_User_2665 · Super Member Joined: May 06, 2006 Posts: 380

The length and weight of a bullet weigh heavy on a matching twist.....that's a law of Physics......noone can argue that.

That said I've had the Law twist itself on at least one chambering by shooting light/short as well as long/heavy...........with crap in the middle.

I've also had at least one shoot outstanding in the middle but nuthin' on the outsides.........

My experienced eye relates that each shooter is different, liking only certain componentry and puking others, some have a bigger profit margin in the black than others thus liking a broader range.......

'Course there is always the rare circumstance............

Crackshot

I have seen my 1-9 twist 223rem shoot 40gr Vmax bullets like there is no tomorrow. And being told that should not happen, have had to prove it. Very Happy

and made a few bucks doing it. Very Happy

It depends greatly on your rifles likes and dislikes, powder charge and what not IMO.

rrogacki

Crackshot wrote:

I have seen my 1-9 twist 223rem shoot 40gr Vmax bullets like there is no tomorrow. And being told that should not happen, have had to prove it. Very Happy

and made a few bucks doing it. Very Happy

It depends greatly on your rifles likes and dislikes, powder charge and what not IMO.

Crackshot,
So you would say that given weights for a particular twist is a myth ??

Crackshot

I think that it depends on the powder charge and the rifle, shoot them all, find out what it likes, Bullet weight and barrel twist is a facter, but can be gotten around with alittle experimenting.

guncollector

As previously noted, a lot of factors effect accuracy. Twist rate in relationship to bullet length and weight is definitely one of the factors and not a myth. For example, a standard AR15 comes with a 1:10 twist and shoots 55 grain bullets great. If you want to shoot heavier bullets, like the 77 grainers I use in competition, you need a faster twist barrel. My competition AR has a 1:8 twist. Try shooting a 77 grain bullet in a 1:10 twist barrel and it will not stabilize. You will find "keyhole" patterns on your target where the bullet went through at an angle off horizontal.

Flint54

Thought I would interject some information into this discussion. I would like to caution against using bullet weight as a determining factor in the selection of a specific twist. The most critical factor is no longer the weight of the bullet, legnth of the bullet is more a factor. That being stated let me explain.

We now have at our disposal bullets filled with tungsten dust or inserts of the same, this raises the weight of a bullet but also shortens the legnth so in actuality this bullet could be used in a slower twist.

On the other hand, we also have quite a selection of monometal bullets, GS Custom, Barnes, J36s etc. These bullets have a longer legnth vs. bullet weight as compaired to standard bullets. These longer bullets require a faster twist. As an example look at the 6MM Remington, the originals used a 1in14 this did not stabilize hunting weight bullets, now with lighter monometal bullets even in the weight that it would shoot they are too long to properly stabilize.

This can be a factor in any caliber, watch your bullet legnth. Cool

Dimitri · Super Member Joined: Nov 25, 2005 Posts: 5946

I've read a long time ago exactly what Flint is talking about. Smile

Bullet Lenght is what determains the twist rate not the weight. Smile

Generally thought the heavier the bullet the longer the bullet hense why you need faster twist rates. This only applies to bullets made of copper and lead, and running at standard velocities as the speed can effect it. The Greenhill Formula is for this.

Greenhill Formula (from stevespages.com/page8e.htm ):

FOR VELOCITIES UNDER 2,800 FPS
twist rate = (150 * bullet diameter squared) / bullet length
bullet length = (150 * bullet diameter squared) / twist rate

FOR VELOCITIES OVER 2,800 FPS
twist rate = (180 * bullet diameter squared) / bullet length
bullet length = (180 * bullet diameter squared) / twist rate

Hope this calculation helps Smile

Dimitri

PaulS

That works for the maximum length of bullet that will stabilize but what about a shorter bullet? Where do you draw the line with a shorter bullet.
A case in point is my 358 Winchester that shoots 158 grain Sierra JHC pistol bullets to .33 inch groups with a muzzle velocity of 2508 fps.
I figured that these bullets would either fly apart (due to speed and rotation) or hit the target in keyhole patterns. To my ammazement they are as accurate and as stable as anything I shoot from it. It shoots the Speer 180 grain bullets almost as well.
I also shoot 110 grain RN bullets from my 3006, that should be way over-sablized, very well too. The '06 has 1 in 10 twist and is most accurate shooting a 165 grain bullet but will shoot 120 grain bullets without any sign of keyholing. Somebody needs to find a better formula for calculating how to stablize a bullet. I am not going to volunteer for this one - my last formula has generated too much controversy for its value and took too long to calculate.

george20042007

The June 2006 issues of Shooting Times addresses this issue on starting on page 54. Chec k it out if you care to.
Keep it coming...

Flint54

Basic facts are that a bullet "CANNOT" be Over Stabilized. This is another one of the Myths. Problems stem from the construction of the bullet itself that leads people to believe that bullets are over stabilized. What happens is that due to very high rotational speeds the core/jacket fail in some form and this causes instability that results in the bullet yawing or blowing apart.

Back in 1979 I got some .35 Remington loading data from J.D. Jones when I was talking with him and Larry Kelly from Magnaport. At the time I was preparing to leave Mt. Clemens, MI for Kodiak, AK. JD gave me some loads that I won't publish but I will say that they would not reach a 35 yard target whole. at 25 yards it looked like a shotgun pattern from the bullet coming apart. Cool

mc223

In the process of developing loads for 223, I discovered that some of my loads shot better groups than some others as would be expected. discounting powder and primer variables.
What stood out the most was when I noticed that some of the lighter and consequently shorter bullets did not group as tightly as the heavier and consequently longer bullets.
It was at about the same time I bumped into the Greenhill formula.I started messing around with the numbers from the loads I had been working on and sure enough there is a direct correlation between the accuracy result I had seen and the actual barrel twist of my rifle.
Even though there may be what seem to be gaps in the actual performance of certain bullets at a given twist rate, I believe the Greenhill formula can be used as a guide to what may be the best place to start when developing loads for a given twist rate.
The Greenhill formula does not care what the bullet weighs, only length and diameter squared are used.
Over stbilization really only comes into play when the target distances are very long as described in an earlier post.
Speed of rotation is relative to time in flight. A 1 in 12 twist creates 300 revolutions in 100 yds. not terribly fast rotation considering the distance covered. if calculated in revolutions per minute(somethig like 34,000 RPM), it makes you wonder why bullets dont just fly apart as soon as they exit the muzzle. Bullet manufacturing and materials have improved a lot since the old days when bullet jackets were too thin for the velocities at all but the slowest twist rates.

Flint54

As an input to this thread Gerard Schultz of GS Custom Bullets in South Africa has a plethora of information on his web site. His Documentation of Monometal/Copper bullets puts Barnes to shame. I even think that Barnes somewhat copied his HV design in the negitive for their Triple Shock line of bullets. Take a look at his data and you can glean much information on the use of this style of bullet.

Links:

www.gsgroup.co.za/faqb...twist.html

www.gsgroup.co.za/07faq.html

www.gsgroup.co.za/hvtech.html

Crackshot

While I have shot Seirra 168gr MHP bullets for years with great results, a few years ago I had a custom rifle put together and was talking to the riflesmith about the barrel, He recomended twist rates and so on and so forth, the twist rate of my M1A came up and he asked If I was shooting 200 or 220gr match bullets in it, I said no,
he was amazed I could hit anything as the 1-10 rate is not meant for (in his oppinion) 168gr bullets in 308 win. But he was the first to say"you never know what a rifles gonna decide it likes"
After I was done with him, I went to the shop and grabbed a box of 200gr Seirra's and they shot like they came out of a sling shot.
I have a AR-15 with a 1-8 twist that shoots 50gr bullets like a lazer.

george20042007

I have to admit this is one subject I never gave much thought to. Since I don't have a custom gun and by off the shelf, I subscribe to trying out many different loads to see "what the gun likes". As most manufacturers build according to whats most likely out there in the way of ammo, and ammo manufacturers do something similar, it boils down to trial & error.

This is the reason why so many of us reload. It's part of the fun & with it comes bragging rights when you come up with the load of loads for a given gun. I do avoid thin jacketed bullets at all costs when it comes to small calibers of high velocities, it just makes sense.

I'd like to know how many of you are like me. Should we give barrel twist more consideration or see what the gun likes?
Keep it coming...