Twist Rates & Bullet Weights........

[TDDude]

Well, I now know that it's the bullet length that matters but that doesn't rhyme.

I was posting on couple other threads on barrel matching and twist rates and was trying to remember what I’d read about this subject and how important it is to match the bullet to the barrel.

I have shamelessly copied this from one of my reloading manuals. The manual is called “The ABC’s of Reloading – 8th Edition” edited by Bill Chevalier.

From chapter 6 called Bullets:

…..but there is a very simple method that works with a pocket calculator or even paper and pencil: the Greenhill formula. The Formula for determining twist rates was the work of Sir Alfred George Greenhil, a mathematics professor at Cambridge University who later served as an instructor at the Woolrich Military Academy from 1876 to 1906. Greenhill Discovered that the optimum twist rate for a bullet is determined by dividing 150 by the length of the bullet in calibers and then dividing again by its diameter. The number 150 is a good choice since it allows a useful margin in the calculations. Most twist rates that are close to the formulated ideal will usually work well. The beauty of this formula is that it works very well for lead or jacketed bullets. Weight does not appear to be that critical a factor. Shape and design do not seem to have that much effect either, up to velocities of 2200 fps and, to a degree, above this. To compensate for increased rotational speed at velocities over 3000 fps, some authorities recommend a slightly reduced twist rate. Although velocity does not appear to be considered within this formula, it is included in the rotation segment in a concealed form. Assume a 1:12-inch barrel firing a bullet at 1000 fps. This equals 1000 rotations per second. At 2000 fps, the rotations per second double. Higher velocity yields a faster spin and is thus considered in the calculations, although it is not specifically mentioned. The most recent interpretations of Greenhill opt for a slightly faster twist with the higher velocity cartridges, in the belief that erring on the side of over-stabilization is better than under-stabilization which may result in a tumbling bullet.
The popular .223 Remington is a good candidate for study. Rifles for this cartridge are currently available with the following twist rates: 1:7, 1:8.5, 1:9, 1:10, 1:12, and 1:14 inches. To apply the Greenhill Formula using the original 55-grain bullet yields the following for one brand of full metal jacket military-type bullet measuring .647-inch in length. The bullet diameter is .224-inch, which, divided into the length of .647-inch, gives 2.89 calibers long. Dividing 2.89 into 150 yields a figure of 51.9, or an ideal twist rate of one turn in 51.9 calibers. Multiplying 51.9 by the bullet diameter (.224-inch) equals one turn in 11.63 inches for this particular bullet.
The original twist for the 223-caliber M-16 rifle is 1:12-inches. In its wisdom (?), the Army decided a heavier (longer) bullet was necessary and the M-16A1 is bored with a 1:10-inch twist. The new military bullet will not stabilize in the older barrels. Bullets as heavy as 70 grains are available for the 223 Remington. For a 70-grain bullet measuring .785-inch in length, dividing by .224 equals 3.5. Dividing 150 by 3.5 equals 42.86, or one turn in 42.86 calibers; then 42.86 multiplied by .224 equals 9.6. Thus a twist of 1:9 or 1:10 is required to shoot this bullet accurately. There are other factors involved, such as the amount of bearing surface on the bullet, velocity and barrel lenth. In some cases, bullets that are not well matched to twist rate can be made to function. For example, a short, 40- or 45-grain bullet, in a 223 with a fast twist of 1:9 or 1:10 inches, will perform if the powder charge is cut back. By decreasing the velocity, you can keep the bullet from tearing itself apart.

Well, there you go. A little math and viola.

[The Annoyed Man]

There are some minor inaccuracies, but all in all it makes some kind of sense. For instance, he says "Bullets as heavy as 70 grains are available for the 223 Remington." I've got some .223 match ammo in my safe right now marked 77 grains, and I've seen catalogs with weights up to 85 grains. So 70 grains is not even close to the high end of .224 bullet weights. Furthermore, the problem with 45 grain bullets in 1:9 barrels isn't that the bullet will tear itself apart (although this could conceivably happen with a hot enough load). The problem is that 45 grain bullet being spun by a 1:9 twist will veer off the proper line of trajectory due to increased spin drift induced by the higher rotational velocity applied on a lighter body, and insufficient bearing surface along the bullet length to stabilize it at that rate. The only centerfire .22 caliber bullets I've ever head of that had a potential problem with bullet disintegration during flight was with the .22-250, loaded up really hot — like around 4,000 fps. That cartridge can push a bullet so fast that it will shed its jacket in flight and come apart in mid-air. But the .223/5.56 is such a small case with such limited capacity that I don't know if you could load it hot enough to disintegrate the bullet.

[glbedd53]

I thought 1 :7 was a faster twist than 1 : 9

[WildBill]

Well, I now know that it's the bullet length that matters but that doesn't rhyme.

I was posting on couple other threads on barrel matching and twist rates and was trying to remember what I’d read about this subject and how important it is to match the bullet to the barrel.

I would think that the barrel length is also a factor and that the manufacturer would optimize the twist to the barrel length using standard military ammo. That is what I would do.

[The Annoyed Man]

I would think that the barrel length is also a factor and that the manufacturer would optimize the twist to the barrel length using standard military ammo. That is what I would do.

It is true that length, twist, and bullet weight all play a part in the equation, but they are not always interdependent. For example...

My son and I both have bolt action rifles in .308 Winchester, with heavy, target grade barrels. His is a Savage 10FP-HS Precision. Mine is a Remington 700 VSF. Both have barrels with 1:10 twists, but his is a 24" barrel, and mine is 26" long. Both perform equally well with 175 grain MatchKings in commercial loadings — which is to say, extremely well. Consider .5 to .75 MOA to be consistently attainable with either rifle. A twist of 1:10 in a .308 is recommended for the heavier bullets in the 175 grain range. Now, for the 168 grain MatchKing, a 1:12 twist is the supposed optimum. However, my 26" barrel will shoot 168 grain MatchKings just the tiniest bit more accurately than my son's 24" barrel. In fact, shooting my very first ever hand loads, I shot the best group I've ever shot in that rifle — putting four 168 grain MatchKings into two holes 3/8" apart, center to center. So in that example, perhaps the additional 2" of barrel is enough to stabilize the 168 grain bullet at a 1:10 twist.

The two most accurate AR15s we have are both varmint rifles with 24" barrels. It is not an exact comparison because mine has a fluted heavy chrome-moly barrel, and my son's has an un-fluted stainless bull barrel; but they will serve for the purposes of this discussion.

Mine has a 1:9 twist. My son's has a 1:8 twist. Mine will shoot 69 grain MatchKings very accurately, but not so much the 77 grain MatchKings. My son's, conversely, will not shoot the 69 grain bullets as well as mine; but it will put five of the 77 grain bullets into .25" at 100 yards. In keeping with Townsend Whelen's maxim that, "only accurate rifles are interesting," my son's rifle is extremely interesting as long as it is shooting 77 grain MatchKings.

Stepping down in bullet weight, his rifle becomes decidedly less interesting shooting 55 grain FMJs, and absolutely boring shooting 45 grainers. Mine, with its 1:9 twist on the other hand, handles the 55 grain bullets better than his, but it still stinks with .45 grain bullets.

And taking things one step further, my son also owns a 16" bull barreled upper with a 1:9 twist. It shoots 55 to 69 grain ammo exceedingly well, but not the heavier or lighter bullets. That barrel is a full 8 inches shorter than my 24" varmint barrel with the same twist, and it performs similarly. This leads me to four conclusions:

1. Twist rate is the most important factor in stabilizing a bullet of a given weight. Generally speaking, faster twist = heavier bullet, slower twist = lighter bullet.
2. Generally speaking the faster the bullet is going, the higher its ballistic coefficient, therefore, barrel length is important to accuracy, but accuracy is not dependent upon barrel length, see below...
3. Barrel length is important for developing velocity. The longer the barrel, up to a point, the longer the charge has to burn before being vented out the muzzle, and the faster the muzzle velocity.
4. Barrel diameter is relevant for maximizing the available accuracy out of any bullet weight/twist rate combination because the fatter the barrel, the more it dampens the harmonics of barrel vibration. And counter-intuitively, a shorter fat barrel is more accurate than a longer barrel of the same diameter because it vibrates with less amplitude than the longer barrel.

Thus, a 16" bull barrel can be more accurate than a 24" heavy barrel (assuming the same twist and bullet weight), but it won't deliver as much velocity. And a 24" bull barrel will be more accurate than a 24" heavy barrel (but not necessarily more accurate than the 16" bull barrel), assuming the same twist and bullet weight, but carrying the 24" bull barrel into the field just may rupture your kidneys. Furthermore, a heavy barrel, shooting the right combination of twist and bullet weight, will be more accurate than a bull barrel shooting the wrong combination of twist and bullet weight — regardless of barrel length.

The conclusion is that A) all of these things are part of the equation, but they are not necessarily dependent upon one another; and B) I am still in pursuit of the perfect rifle, but what a ride it's been!

[WildBill]

I would think that the barrel length is also a factor and that the manufacturer would optimize the twist to the barrel length using standard military ammo. That is what I would do.

It is true that length, twist, and bullet weight all play a part in the equation, but they are not always interdependent. For example...

I believe that they are interdependent. You are throwing other variables into the equation, i.e. different manufacturers, different barrel types, etc. Regardless of the length, twist or ammunition type, a varmit barrel made by Winchester is going to shoot differently that a fluted barrel made by Remington. It's not just a matter of twist and length. Barrels will have different vibration characterics that will affect accuracy. Different types of actions, stock materials and bedding will all effect accuracy.

Stepping down in bullet weight, his rifle becomes decidedly less interesting shooting 55 grain FMJs, and absolutely boring shooting 45 grainers. Mine, with its 1:9 twist on the other hand, handles the 55 grain bullets better than his, but it still stinks with .45 grain bullets.

You will never develop a formula that explains your results. I suggest that you don't shoot 45 grainers. It will just make you angry. Good luck on your quest for "the perfect rifle." I suspect that you already know that it doesn't exist. But that doesn't mean you can't have fun looking.