Home Forums Hunting & Shooting Ask The Gunwriters Stability - Bullet Length or Weight

Specifically , If a barrel is twisted too loose to shoot a 90 gr. Scirroco accurately , Would you expect different results with a monolithic bullet of lighter weight but same length ?

Length is the factor, not weight.

Back when all bullets were mostly lead (some wearing a copper condom), we fell into the habit of using the familiar weight labels when we discussed stability. It worked, because length changed with fair predictability as weight changed.

That's no longer true. One recent example from my own bench: The new Barnes copper-tin matrix Varmint Grenade bullets are quite long for their weight. The 36-gr .22-caliber bullet is almost as long as some 50-grain lead cored bullets. And, wouldn't you know, those 36ers aren't fully stable in my 1-16" twist Hornet that shoots all 45s perfectly well. It is length, not weight.

As a real-life example, consider a wooden toy top. A short squat little turnip-shaped top will spin upright and stable even at slow spin rates. But you can't spin a pencil of equal weight fast enough to get close to having it stay upright.

cd4570

It would be less stable with the lighter bullet of same dimensions.

Just change the wt of the bullet in the following link. Pretty simple.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

You can prove that to yourself by throwing a nerf football vs a heavier one of about the same dimensions.

If you use the Greenhill formula as a rule of thumb, you can figure, more or less, what the "optimum" bullet length for a particular rate of twist.

T = (150 x (D x D))/L
D = Diameter of the bullet in inches
L = Length of the bullet in inches
T = Rate of twist in inches

Jeff

The Greenhill formula is full of holes, mostly because it was developed for lead bullets back in the late 19th century. It can provide only very approximate results and even then some people suggest the 150 constant in the equation should be raised to 180 for higher velocity bullets.

There are too many better ballistic programs available today for figuring twist to mess with the Greenhill.

Can you point me to these better equations? I'd like to compare them to the Greenhill results.

Jeff

Sierra has one on their ballistic program, and my friend Don Miller published a very detailed equation in PRECISION SHOOTING, I believe. Don's formula is also included in Bryan Litz's book, and I believe in the computer CD program that's included with the book.

I kept using Greenhill until maybe 3 years ago, before updating my computer programs.

This program at JBM Ballistics utilizes Don Miller's works in that shortcut calculation for stability.

I believe that the actual paper/article from Precision Shooting is there as well.

Weight, length vs caliber, and twist rate seems to be the deciding factor. My experience is that lead-free powdered metal core bullets of longer weights i.e 50 grain .224" or 62 grain .243" will require a twist rate 1 to 2 steps faster than lead core bullets for maximum stability and accuracy. A 1:10" for the .22-250, a 1:9" for the .223 Remington, and a 1:8" for the .243" bullet regardless of cartridge. Totally theory based on the Miller Calculator, but it is borne out empirically in shooting. If lead free bullets become the norm, then factories will probably be forced to increase twist rates.

Unfortunately, copper and copper/tin is lighter than lead, so that any bullet in a standard weight is going to be longer than a lead core bullet. The same is true for monolithic copper bullets, but to a slightly lesser degree based on my experience.

A fine gentleman by the nickname of Unclenick on The FiringLine Forums has an excel spreadsheet available at his file repository, http://drop.io/unclenick. I believe it's called "Bullet stability and twist estimator". It is based on Don Miller's March 2005 Precision Shooting magazine article. The bullet gyroscopic stability factor "s" must equal 1 or higher for a bullet to be stable, with 1.5 being "optimal".

I've found it easier to use without having to do precise measuring of bullets such as when you use the JBM ballistics site (although that's an excellent resource in it's own right).

Second question : I don't recall twist rates coming up in a lot of conversations until folks started wanting to shoot the heavy .224 bullets , primarily in the TTH . So , given Nosler's claim that a bullet can't be " over stabilized " why don't we use tight twists for all cartridges in all bullet weights ?

There must be some advantage to loose twists in some applications . To put it another way , would my 22 250 AI still shoot the 50 gr Sierra Bltzkings into little bitty groups if I put a 1 in 9 barrel on it , replacing the 1 in 12 it now has ?

It's purely an academic question - I ain't changing a thing !

Up to a point, but you CAN overtwist a bullet, for example, neither of the 1-9" twist Savage .223s I own can shoot a Hornady Super Explosive bullet, or a Sierra Blitz, they twist the thin-skinned bullets apart.
For best accuracy, according to most benchresters I know, you should use the slowest twist barrel that will stabilize the bullet you intend to use. That's why almost all benchrest shooters use 1-14" twist barrels in their 6PPCs, that twist will stabilize the 68gr. bullets they tend to favor, but not overtwist them.
Unfortunately, there ain't no free lunch, and compromise is usually the "Blue Plate Special", to mangle a metaphor.

An over-stabilized bullet may shoot really well, but it tends to ride more nose-up than a just-stabilized. This reduces the BC somewhat at longer ranges.