Topic of interest: No. 16
Bullet stability depends primarily on
gyroscopic forces, the spin around the longitudinal axis of the bullet imparted by the
twist of the rifling. Once the spinning bullet is pointed in the direction the shooter
wants, it tends to travel in a straight line until it is influenced by outside forces such
as gravity, wind and impact with the target.
Rifling is the spiral or helix grooves
inside the barrel of a rifle or handgun. These grooves were invented a long time ago,
perhaps as early as the 14th century. However, the smooth bore, using the round
ball, was the choice of weapons for warfare even through the American Revolutionary war.
The smooth bore musket could be loaded faster than the rifle and didnt foul, as bad,
with the combustion products of black powder.
The rifling grooves helix is expressed in a
twist rate or number of complete revolutions the grooves make in one inch of barrel
length. A 1in10 or 1:10 would be one complete turn in 10 inches of barrel length.
How important is twist rate? David Tubb, a
winner of several NRA High Power Rifle Championships, was using a .243 rifle with a 1 in
8.5 twist barrel. He wasnt able to get consistent accuracy until he changed to a
rifle barrel with a 1 in 8 twist. The ½" twist change made all the difference
between winning or losing the match.
A term we often hear is
"overstabilization" of the bullet. This doesnt happen. Either a bullet is
stable or it isnt. Too little twist will not stabilize the bullet, while too much
twist, with a couple of exceptions, does little harm. Faster than optimum twists tend to
exaggerate errors in bullet concentricity and may cause wobble. The faster twist also
causes the bullet to spin at higher rpm, which can cause bullet blowup or disintegration
because of the high centrifugal forces generated. For example, the .220 Swift, at 4,000
fps., spins the 50-grain bullet at 240,000 rpm.
One of the first persons to try to develop
a formula for calculating the correct rate of twist for firearms, was George Greenhill, a
mathematics lecturer at Emanuel College in Cambridge, England. His formula is based on the
rule that the twist required in calibers equals 150 divided by the length of the bullet in
calibers. This can be simplified to:
Twist = 150 X D2/L
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
This formula had limitations, but worked
well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic
experts suggest substituting 180 for 150 in the formula. The twist formulas used in the
Load From a Disk program, featured at this web site, uses a modified Greenhill formula in
which the "150" constant is replaced by a series of equations that allow
corrections for muzzle velocity from 1,100 to 4,000 fps.
The Greenhill formula is simple and easy to
apply and gives a useful approximation to the desired twist. The Greenhill formula was
based on a bullet with a specific gravity of 10.9, which is about right for the jacketed
lead core bullet. Notice that bullet weight does not directly enter into the equation. For
a given caliber, the heavier the bullet the longer the bullet will be. So bullet weight
affects bullet length and bullet length is used in the formula.
To measure the twist of a barrel, use a
cleaning rod and a tight patch. Start the patch down the barrel and mark the rod at the
muzzle. Push in the rod slowly until it has made one revolution, and then make a second
mark on the rod at the muzzle. The distance between marks is the twist of your barrel.
To see how this works out, assume you
bought a .222 Remington rifle and you measured the twist rate as described above. The
twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain Speer SPS and
the 50-grain Hornady SX. The Speer bullet measures .812 inches in length and the Hornady
measures .520 inches. Using the formula above we calculate the following twist rate:
Speer 70-grain: 1 in 9
Hornady 50-grain: 1 in 14
These calculations show that the 50-grain
bullet will be stabilized, but the 70-grain wont. Sure enough, when you try these
bullets out, the 50-grain shoots ¾ MOA while the 70-grain wont group on the paper
at 50 yards. Twist is important!
Watch our web site for the next topic of
interest "What is Recoil and how is it Calculated." Until then, shoot safely and
know where you bullets are going.