Factors in bullet stabilization

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ELMOUSMC

New member
I would like to understand the factors that are in envolved in stabilizing a bulletin a previous post I was having issues with my .22 hornet,with information from members on this forum most of them were solved.iam still not able to get any 45 grain bullets to settle in to any sort of accuracy and I am convinced it is a stability problem.My rifle has a 24 in barrel with a 1 in 14 twist any ideas
 
Your bullets .223" or .224"? 1 in 14 is pretty typical for the Hornet.
What load are you using? What rifle? Could be the rifle is not up to great accuracy.
45 grain bullets out of a Hornet supposedly are the most accurate at roughly 2500 fps and up.
 
The main factors are:

Bullet length
Bullet mass
Bullet spin rate, which is a function of velocity and rifling twist.
Air density
Velocity

Try using Geoffrey Kolbe's rifling twist calculator. You want to get a gyroscopic stability factor of at least 1.3 for hunting accuracy and 1.4 for match accuracy under standard conditions. He recommends 1.5 to cover changes in weather better. You can use the output graph to see where your bullet lies in the range.

Another stability calculator is at the JBM site. It uses a different method of approximation. That site also has some handy bullet length tables. I have found, as a practical matter, though, that calculator at JBM underestimates stability of .22's a little, where it is just fine with most .30 caliber bullets. I don't know why.
 
stability

The rifle is a savage model 40 bull barrel with a 1 in 14 twist the rounds are 45 grain HP bee's mads by Barnes when shot over a chronograph they show
2480-2500 fps-the brass is PPU and Iam using 10 grains of Lil'gun
 
ELMOUSMC said:
the rounds are 45 grain HP bee's mads by Barnes
Click to expand...

That's your problem. The only Barnes bullet I find on their site at 45 grains is their TTSX, which is 0.698 inches long. Barnes bullets are solid copper, which has a specific gravity (SG) of 8.89, where jacketed bullets have combined core and jacket SG typically around 10-10.4. As a result, your bullets are longer for their weight than a jacketed bullet. Greater length gives air pressure a longer lever arm against the nose to try to flip the bullet over with, and more centripetal inertia to pull at the nose as it nutates around trying to find a stable yaw condition. You need more mass from greater density to have enough gyroscopic stiffness from the spin to fight that. Bullet length is the single most influential factor in finding twist rate, not weight. Tables showing weight and stability all assume a standard jacketed lead core bullet shape. These days we often have something that is outside that standard, like your copper solids or a long VLD shape that needs more spin than the standard assumed bullet does.

Both of the stability calculators I linked to show that in ICAO standard atmospheric conditions —59°F and 29.92 inches mercury and zero humidity (a worst case assumption)—the bullet needs a twist of about 11.0" to 11.5" to have good stability at your velocity. The second calculator is getting the stability factor about 13% low, as it tends to with .22's, but even allowing for that, you have an unstable bullet right about at the dividing line between stable and unstable (where the gyroscopic stability factor is 1.0). The first calculator, which is the better of the two, as it considers more bullet details, shows that at 2500 fps the 14" twist gives a stability factor of about 1.01. In that range, the bullet will go down range and never quite tumble, but never stop seeking a stable point, so the initial yaw never settles out.

Any stability factor under about 1.4 produces bigger groups and under 1.3 they can start to get significantly bigger. Below 1.0 you have outright tumbling and the bullets start to keyhole and can miss the paper completely at longer ranges.

Bottom line here is that your bullet choice is just not suitable for your chambering and twist rate. If it were a jacketed bullet (the only kind in your caliber, other than lead bullets, that were available when your chambering was invented) it would be about 0.6 inches long at 45 with the same nose and base form, and would have a stability factor of about 1.7 and be flying just fine.
 
bullet stability

After using the stability calculators the 45 grain bee is unstabile at the slower fps only marginal at best at higher fps.I guess that I will be keeping my bullet weight at 40 or below
 
It's not the weight that is the principle issue, it's the length. Run the calculator and keep changing the length a little bit at a time. You will find a length for 45 grains that is just fine, as long as your bullet choice doesn't exceed it.
 
That's the ICAO standard atmosphere. The ICAO is the International Civil Aviation Organization, so it's an aircraft standard. It is 15°C (59°F), barometric pressure of 760 mm (29.92") mercury (Hg), and 0% Relative Humidity (RH). Ballistics here used to use U. S. Army Standard Meteorological Conditions (aka, Army Std. Metro), which were for the same temperature, 59°F, but with 29.53" Hg, and 78% RH. Both the lower barometric pressure and higher humidity of Army Std. Metro make the air less dense (water vapor is less dense than dry oxygen and nitrogen), lowering the amount of spin needed for stabilization a little. Brian Litz told me in an email that Army St. Metro is now considered obsolete in the firearms industry, so all the BC's you see listed for bullets are from measurements that were adjusted for the ICAO standard atmosphere before determining BC from them.

As to why the aviation folks prefer the more dense 0% RH, I can only speculate. More dense air will increase the amount of fuel you use, and since planes can fly into very different weather conditions than they start out in, a worst case fuel allowance calculation makes the most sense to use.
 
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