rate of twist and 223 and 308

[bluebayou]

Are there any kind of general rules for bullet stability and rate of twist?  It seems that a faster twist is needed for bigger bullets most of the time.  I am looking for a .223 bolt action for less than $500.  The Savage is the only one that has a fast twist at 1:9.  Everyone else, Remington, Howa, Winchester, etc have a 1:12.  I want the option to shoot heavy and light bullets.  
My HandiRifle in 223 will only shoot light (50 grains or less) and it is 1:12.  Is this because of the poor chamber of the HandiRifle or because of the 1:12 twist.
My other personal experience is with my poor, experimented-on Rem 700 in 308 that will shoot 165 and 168 gr all day long but throws lighter (150 grain) bullets out to 3-5" groups.  It has the 1:10 twist.

So, I guess the nitty gritty is---will a 1:12 twist stabilize a 60 plus 223 bullet and should a 1:10 twist stabilize a lighter 308 bullet?  I assume that there are all kinds of variables here, but is there a "rule" to this?

[Iowegan]

Bullet stability comes from the rate of spin vs the properties of the bullet. Properties would include length, weight, diameter, and shape. The rate of spin is determined by the barrel's twist rate and the muzzle velocity.

Assuming the rate of spin is adequate, the bullet will go down range very stable. As air resistance impinges on the bullet, it will slow down in velocity and in spin rate. Once the spin rate drops below the stability rate, the bullet will start to wobble and will soon become so unstable that it will tumble.  A sure sign if instability is when you see oval holes or worse yet key holes in your target.

If you shoot a target at 300 yards and the holes are nice and round, you probably don't have a stability problem. Many other things can cause accuracy problems. In other words, bigger groups probably aren't from instability.

A 1:12 twist in a 223 Rem should stabilize a 62 grain bullet out to 500 yards.  Your 1:10 twist in the 308 should work well up to 180 grains.

A twist that is too fast for the bullet will strip the jacket and cause accuracy problems. That's why your 150 grain loads are not very accurate. If you slow them down to 2400~2500 fps, they will probably shoot much tighter.

[jvs]

This and the following post may help answer your question.  I will add the URL's for each website if you want to read more.

The Rifle Barrel
By Chuck Hawks

Rifling twist rates

The rate of twist, expressed as one turn in so many inches (i.e. 1 in 10"), is designed to stabilize the range of bullets normally used in a particular caliber. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight, and heavier bullets require faster twist rates than lighter bullets of the same shape. It is undesirable to spin a bullet a great deal faster than necessary, as this can degrade accuracy. A fast twist increases pressure, and also the strain on the bullet jacket.

Fortunately, the rate of twist chosen by the rifle maker is usually appropriate for the intended cartridge. Anyone ordering a new barrel for a rifle will generally do well to specify the standard twist as supplied by the major rifle manufacturers for that caliber.

Once in a great while, though, a manufacturer makes a mistake. One such case involved the .244 Remington. When first introduced, barrels for this caliber were made with a 1-in-12 twist, because Remington anticipated that their new cartridge would be used primarily for varmint shooting. The 1 in 12" twist is ideal for best accuracy with varmint weight bullets (70-85 grains) in a high velocity .24 (6mm) caliber rifle. The heaviest spitzer bullet that a .244 with a 1 in 12" twist barrel could stabilize was 90 grains. The customers, however, also wanted to use their new .24 caliber rifles for hunting medium size big game, with 100 grain bullets. Needless to say, customers ignored the new .244 Rem. Remington soon saw the error of their ways, and changed the rifling of their .244 barrels to 1 turn in 9", but the damage was done. Sales remained so slow that eventually Remington had to discontinue the .244. The following year they reintroduced the exact same cartridge as the 6mm Rem., and produced all 6mm rifle barrels with 1 in 9" twist barrels, which can stabilize all .24/6mm bullets. This solved their marketing problem, and 6mm rifles began to sell.

The usual twist rates for some of the more popular rifle calibers are given below. (For a more comprehensive list, see "Common Rifle Barrel Twist Rates" on the Rifle Information Page.)

.22 Short = 1 in 24"
.22 Long Rifle = 1 in 16"
.223 Remington = 1 in 12"
.22-250 Remington = 1 in 14"
.243 Winchester = 1 in 10"
6mm Remington = 1 in 9"
.25-06 Remington = 1 in 10"
.257 Wby. Mag. = 1 in 10"
6.5x55 Swedish Mauser = 1 in 7.5"
.260 Remington = 1 in 9"
.270 Winchester = 1 in 10"
.270 WSM = 1 in 10"
7mm-08 Remington = 1 in 9.25"
7mm Rem. SAUM = 1 in 9.25"
7mm Rem. Mag. = 1 in 9.25"
.30 Carbine = 1 in 16"
.30-30 Winchester = 1 in 12"
.308 Winchester = 1 in 12"
.30-06 Springfield = 1 in 10"
.300 WSM = 1 in 10"
.300 Win. Mag. = 1 in 10"
.300 Wby. Mag. = 1 in 10"
.303 British = 1 in 10"
.32 Win. Spec. = 1 in 16"
.338-57 O'Connor = 1 in 10"
.338 Win. Mag. = 1 in 10"
.35 Remington = 1 in 16"
.350 Rem. Mag. = 1 in 16"
.375 H&H Mag. = 1 in 12"
.416 Rem. Mag. = 1 in 14"
.444 Marlin = 1 in 38"
.45-70 Govt. (Marlin and Ruger rifles) = 1 in 20"
.450 Marlin = 1 in 20"
.458 Win. Mag. = 1 in 14"

Not all rifle barrels of the same caliber have the same twist rate. A fellow ordering a custom rifle may have his own ideas about twist, as may the builder. For example, some .270 Win. barrels are rifled with a 1 in 12" twist, some .30-06 barrels are also rifled 1 turn in 12", and some .300 Magnum barrels are rifled 1 turn in 14". Usually these variations make no great difference. These slower twists may give slightly lower pressure, as well as very slightly better accuracy with the lighter bullets in each caliber. They will still stabilize the heavy bullets over practical hunting ranges. However, they might not be such a good choice, or quite as accurate, for shooting heavy bullets at extreme range (like 600-1000 yards).

 http://www.chuckhawks.com/index2d.rifles.htm  

[jvs]

The Heart of the Rifle
By Mark Ducros Stouse


Understanding Basic Custom Barrel Terminology

Today's barrel market has never been more active.  There are
approximately nine primary makers of first-quality custom barrels,
representing a variety of production techniques: single-point hook-cut
rifling, multi-point broach-cut rifling, and button rifling.  While each production method is capable of turning out a very accurate barrel when properly done, all three have their die-hard adherents and critics.

Before we begin a brief discussion of rifling types, it is important to
establish a short glossary of terms.

In a rifled bore, there are two dimensions: the land dimension and the
groove dimension.  In a rifled barrel, land are narrow, ridge-like features that run the full length of the barrel.  Typically, there are six lands separated by six grooves in a modern rifle barrel, but many are made with as few as three lands and as many as ten.  The purpose of the land is to grip the bullet as it travels down the bore, and impart a specified rate of spin designed to stabilize the bullet in flight.   Think of the way a quarterback throws a football, and you'll get the basic idea.

In a .30 caliber barrel, for example, the land dimension gages around .300", while the groove measures close to .308".  From this, one can see where some .30 caliber cartridges get their names, e.g. the .308 Winchester and the .300 Weatherby.   Both are .30 caliber cartridges, but their designers emphasized one dimension or the other according to what sounded better in marketing literature.  Regardless of the cartridge name, however, the groove dimension is typically the same size as the bullet that will pass through the barrel.

The second term that is important to understand is the rate of twist.
Expressed in terms of the number of revolutions per inch of barrel length, the twist rate dictates what size bullets the rifle will be able to shoot accurately.  For example, the most common twist rate for a .308 caliber barrel is one complete revolution every ten inches.  This ratio is commonly expressed by a shorthand designation such as 1:10 or 1/10.  Other barrel makers will express the twist in terms of the number of inches, e.g. "You need a .308 caliber, 10 twist."

A good simplified rule of thumb is that the heavier and longer a bullet is, the faster the twist rate needs to be to stabilize it in flight.  For example, a 1:12 twist will stabilize .308 caliber bullets weighing up to 165 grains, particularly if bullet velocity is reasonably high.  In order to shoot heavier .308 caliber bullets in the 170-220 grain range, you would need a 1:10 twist.

There is another little wrinkle in the issue of twist rate, and that is the interaction between twist and bullet velocity.  Let's examine two .30 caliber cartridges that illustrate this relationship: the .308 Winchester and the .300 Winchester Magnum.  To review, the .308 Winchester is a short cased cartridge holding a moderate amount of powder, while the .300 Winchester is a large-volume, long-case round.

To shoot a 155-grain .308 bullet accurately in the .308 Winchester with
full-house loads, you can use a barrel with a 1:12 twist.  To shoot the same bullet in a .300 Winchester, however, a slower rate of twist - probably 1:14 -- would be required to avoid the yawing and pitching associated with over-stabilization problems.    Now, let's look at a heavier bullet like a 220-grain Nosler Partition.   The .300 Winchester, with its higher velocity potential, can shoot this weight of bullet with a 1:10 or 1:12 twist barrel without any problems.  Load the same 220-grain bullet into a .308 Winchester, however, and you had better have a barrel with a faster twist. The reduced velocity of a .308 Winchester would demand a 1:8 or 1:9 twist to stabilize such a heavy bullet in this cartridge.



 http://www.xtremeaccuracy.com/Rifle_Barrel.htm