The better the bullet is, and the closer the center of mass of the bullet is to the lands center, the twist rate should be faster for better accuracy. The OAL of round with the longer bullets may be placing you closer to the lands. If you load to the same OAL for the lighter bullets, you will find that any caliber is more accurate as long as you have enough bullet to seat in the neck. 2.545 is the OAL I use on for .223 rounds in my AR, which places it approx 0.15 off the lands. I do not know what the needed length for a handi is, but this can definitely effect accuracy. Also the gyroscopic stability factor, Sg. Shorter bullets typically have a lower Sg because they have a lower moment of inertia ratio. The Iy/Ix ratio is driven by the length of the axis of the bullet (Iy) when comparing the same caliber with the same weapon since the axis of the weapon (Ix) will be unchanged. Your success with the longer bullet could be affected by the OAL issue or the Sg factor of a better bullet. As far as the length vs. weight issue take a look at info from Sierra. Also I attached some info from Military Studies on twist rates and 5.56mm.
This is very interesting to me, what is the OAL of each of these rounds, maybe the OAL can compensate for the lack of twist, or maybe it is the quality of the bullet?
APPROXIMATE GYROSCOPIC STABILITY OF TYPICAL RIFLE AMMUNITION
(Calculations based on Army data)
*Values estimated for twist rate implied by Greenhill; S=gyroscopic stability factor 1.5-2.0 optimum
5.56mm M193, 55-gr. FMJ BT, velocity 3200 f.p.s. (MI6AI/A2 with 20” bbi.)
14” twist 12” twist 10” twist* 7” twist
S=1.0 S=1.4 S=2.0 S = 4.1
MARGINAL (M16A1) (Mini-14) (M16A2)
(XM16) V=3080 USMC
5.56 mm NATO M855/SS1O9, 62-gr. FMJ BT, velocity 3050 l.p.s. (M16A1/A2 with 20” bbl.)
12” twist 10” twist 81/4” twist* 7” twist
S=0.85 S=1.2 S=1.8 S = 2.5
UNSTABLE MARGINAL (NATO Std.)
(Ml 6A1) (Mini-14) M16A2
Stabilizing High B.C. Bullets
07.16.2003
Q: Are bullets with a higher ballistic coefficient more difficult to stabilize?
A: Presuming that you are asking about bullets in a specific caliber, like .224, .264, .308, etc, compared to bullets of lower ballistic coefficients in the same caliber. And the answer is generally yes. If the caliber is given, there are just three ways to make the ballistic coefficient higher. One is to increase the weight of the bullet. Another is to make a sharper but longer point on the bullet. And, if the bullet does not have a boat tail, that can be added to reduce tail drag.) Of course, these methods can be and are used together to manufacture bullets with high ballistic efficiency.
There are many factors which affect the stabilization of a bullet. Some of these have to do with the mass properties and geometrical properties of the bullet design. Others have to do with aerodynamic properties and spin rate of the bullet. From the science of advanced dynamics we know that there is a limit to the ratio of the transverse moment of inertia to the polar moment of inertia of any spin-stabilized rotationally symmetrical body, such that beyond this limit the body is at best marginally stable and easily destabilized by small disturbances during flight (such as a small gust of wind). This in turn sets a practical limit to the length of the bullet compared to its diameter, and hence to the weight of the bullet, since if the diameter cannot change, weight cannot increase unless the bullet becomes longer. This isn't a hard limit, but a practical rule of thumb is to worry about stability if the bullet length is more than about 5 calibers.
We also know that when the separation between the center of mass and the center of aerodynamic pressure (which is forward of the center of mass) becomes larger, the bullet becomes more susceptible to aerodynamic disturbances, making it harder to stabilize. This happens when bullet points get longer. It especially affects "low drag" bullet designs.
A bullet is stabilized by spinning it, and higher spin rates are required for heavier bullets with longer points (and higher ballistic coefficients). This is reflected in the barrel twist rate recommendations for certain bullets in Sierra's line. These bullets all have very high ballistic coefficients compared to lighter and more blunt bullets in their calibers.
One of the best practical references on the subject of spin stabilization is an article that is now over 40 years old. This article is, "Truth about Twist" by John Maynard in the 16th edition (1962) of the Gun Digest, and reprinted in the Gun Digest Treasury published in 1966. There is also an excellent and complete treatment of this subject in Modern Exterior Ballistics by Robert L. McCoy, but it is highly mathematical.
Bill McDonald
