been reading on this a bit and am more confused than before my 308 has a 1 in 10 twist would a 150 grain be good or would heavier be better have always used sierras but just got 8 boxes of nosler ballistic tips 150 given to me they seem to group pretty good when temp is cool been loading with varget but fixing to try imr 4064 to see which is best . the 308 groups in .5 groups sometimes then it opens to 1 moa and then may go to 1.5 groups . is getting frustrating

That 1-10 should handle just about anything you want to run down it.

Groups opening could be looseness in the screws, barrel heat, barrel channel rub or numerous other little things.

Check the clearance on the barrel with the stock, check the action screws, once your sure things are good there check your best load. If you still have issues with it while shooting progressive rounds it could be the recoil lug area is not making good contact. IF it is simply one load shoots excellent and the next opens up, could be seating depth or primers.

Either way the twist should not be the issue. To an extent it is better to be over stabilized than under.

Good luck.

It's LENGTH that determines the minimum twist rate that will stabilize (Stabilized = keeping it traveling point forward, IE: NOT keyholing) the bullet.
IIRC Sierra claims taht their 220gr MK will stabilize when launched at 308Win velocities.
Weight effects the velocity you can get from a given case capacity
1-10 has been the "standard" twist rate for a 30cal, ever since the days of the 30-40 Krag launching it's 220gr RN bullets.

well then my problem isnt the bullet it doesnt keyhole . tho i had a 30.06 keyhole at 1 time . thinking it mayber the recoil lug as this is the stock synthetic stock checked torque it was loose then torqued and it was a lil better but still not good

I do not mean to high jack this post as I feel this is related: Is there a computable relationship involving twist, bullet length, and caliber? Or is every caliber different? Seems there might be a constant somewhere.

quote:

Originally posted by Ole Miss Guy:
I do not mean to high jack this post as I feel this is related: Is there a computable relationship involving twist, bullet length, and caliber? Or is every caliber different? Seems there might be a constant somewhere.

Yes there are computable twist rates base upon bullet length per caliber, and programs available that will do this for you.

Some calibers are overlapping depending on how closely they are in diameter for length.

The constant as was mentioned is the longer the bullet for caliber generally the faster twist required to stabilize it. Velocity can also effect this to some degree as well.

Fun stuff huh.

Old Miss Guy
The "formulia" uses caliber as it's measuring unit. IE a bullet length of x calibers
Do a search on "the greenhill formulia" (that's what everyone bases their charts on). Remember it dosn't give you the optimum twist rate, rather the minimum twist that will stabilize a given length of bullet (understandable, as it was developed to determine the longest bullet that could be fired from an existing barrel)

quote:

Originally posted by Ole Miss Guy:
I do not mean to high jack this post as I feel this is related: Is there a computable relationship involving twist, bullet length, and caliber? Or is every caliber different? Seems there might be a constant somewhere.

As Tailgunner suggested.


Greenhill Formula for Rifling Twists from Engel Ballistic Research

The Time-honored Greenhill formula was originally used for determining twist rates in the new rifled artillery in the 19th Century.. It is important to note that it is projectile *length*, not *weight* which is the determining factor when considering the best twist rate for your intended use. Weight can vary for a specific bullet length from use of different material and also is determined to some degree by projectile shape. Often long-range match shooters utilizing low drag or VLD projectiles will voice a preference for "lazy" twists; i.e. just enough to stabilize the bullet. The requirements of a Shooter who is utilizing both high velocity and Subsonic ammunition are quite different.

When utilizing subsonic ammunition there is another factor to take into consideration. The projectile not only has only (about) 1/3 of the forward velocity of standard ammunition in Rifle calibers, it also has 1/3 of the *rotational* velocity...think "RPM's". This makes for a less gyroscopically-stable projectile, so a faster rate of twist is indicated than by simply applying Greenhill alone. This wisdom was imparted to me in a conversation with the great barrel-maker Boots Obermeyer. It makes perfect sense, and doubly so coming from him, so we'll take it as Gospel.

Greenhill Formula For Rifling Twists

T*B=150*Sqrt (Density of Lead/Density of Bullet)

T=Twist

B= Bullet length

Both units have to be in 'Calibers', density portion is optional

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To determine the *approximate* twist rate for a given projectile (this varies a bit by shape):

Using the a 190 gr Sierra MatchKing bullet #2210M (.,30 cal) as an example. If we measure the bullet's length it is 1.375" long .

B=Bullet length in *Calibers* (Length/Diameter), hence 1.375/0.308= 4.46 *calibers* long

From formula we have T=150/4.46=33.6. This figure is now in *calibers*, so we convert the twist from calibers to inches. So T=33.6 x 0.308=10.34".

This is why most commercial rifles in .300 Win. Magnum come with a 1:10" twist barrel.

***********

One can go further and get involved in formulas for density, but for most conventional lead/gilding metal projectiles, the above will suffice. Note that this is for a normal High Velocity loading. Subsonics generally require a somewhat faster twist rate. If one plans to use both High Velocity and Subsonic loads in the same platform, a good compromise which works acceptably for both must be found. If the platform is a "dedicated" gun in which subsonic ammunition alone will be used, then a faster rate can be utilized in order to be able to accurately shoot longer (and thus heavier) projectiles subsonically and with good accuracy.

Example:

Twist rates for 7.62mm/.308:

High Velocity only: 1:11-1/8" to 1:12"
Both: 1:10"
Subsonic ONLY: 1:8"

Bear in mind that "dedicated" back at the shop might not be "dedicated" in the field. Most knowledgeable end users prefer to have the option of utilizing both loads so as to be able to address changing tactical requirements on the spot

you have the rule and then the exception to the rule. A 1-in-16 hornet barrel will stabilize a 60gr flat base spire point bullet. Go figure! (Probably due to low muzzle blast?)