Everything You Wanted to Know About Rifling …
… but were afraid to ask
Savage 1920 .250-3000 (left); Kimber Montana .223.
Obvious differences in the newer rifle are stainless steel,
synthetic stock, optical sight. Not so obvious is the rifling twist
— 1-14 in the Savage, 1-9 in the Kimber.
I don’t really care “who put the ram in the ‘rama lama ding dong’” but I would like to know who invented the concept of spinning a projectile so it would fly more consistently and accurately. Actually this is one concept we can see. Major league pitchers grip a baseball so it will spin and fly more accurately.
These 75-grain bullets work fine but it is hard to find 100-grain bullets short enough to work with the slow twist.
Knuckling Down
What if the baseball doesn’t spin? A few major league pitchers had successful careers by learning how to NOT make the ball spin. The pitch is called a knuckle ball after the way it is gripped. When a ball doesn’t spin, it moves around, randomly and unpredictably. No one knows exactly what it will do, which is why knuckle ball catchers use an oversize mitt and batters get so frustrated by a well-thrown knuckle ball. Of course, if the ball does spin it’s just a slow, predictable pitch and any major league hitter will kill it.
For many years, centuries really, muskets were smoothbore and fired round lead balls. They were reasonably accurate to 75 or maybe 100 yards but would soon start to move unpredictably. Military doctrine in those days was for highly drilled musketeers to fire in volleys. Musketeers didn’t fire at individual targets but at enemy formations. A common tactic was to fire a volley followed by a bayonet charge.
Even experienced shooters often confuse the terms “caliber” and “cartridge.” People ask “What’s your favorite caliber for (fill in the blank — deer, moose, elk, bear).” “Hmmm, let’s see; probably 30 caliber.” “But which one? It could be .30 Carbine, .30-30 Win., .300 Savage, .30-’06, .300 Win. Mag.” “So it’s not caliber you are asking about, it is what is your favorite cartridge. Not the same thing.”
A typical American 30-caliber barrel has a hole drilled through it measuring a nominal 0.300″, or 300/1,000th of an inch. This is called the bore diameter. To make it a rifle barrel, spiral grooves are cut in the steel. For a typical 30-caliber bore, the grooves are cut 0.04″ (4/1,000 of an inch) deep. The remaining uncut steel spirals, called lands, are still bore diameter. It is these lands that engage the bullet and impart spin to it.
Measuring from the bottom of a groove to the bottom of the groove on the opposite side of the barrel, the total distance is 0.08″ greater than the bore diameter. The total distance, 0.308″ in this example, is called the groove diameter. Generally bullet diameter is the same as groove diameter.
Smaller diameter barrels generally have shallower rifling grooves. For example most American-made centerfire .22 cartridges (e.g. .218 Bee, .219 Zipper, .220 Swift, .221 Fireball, .222 and .223 Rem., .224 Wby, .225 Win.) have a bore diameter of 0.218″. Rifling grooves are cut to a depth of 0.03″ so groove diameter is 0.06″ larger than bore diameter, or 0.224″.
Above .22 caliber to .27 caliber, American-made barrels generally have the grooves cut to a depth of 0.0035″ making groove diameter 0.007″ greater than bore diameter. For example the .243 Win., 6mm Rem., 6mm CM, and .240 Wby all have bore diameters of 0.236. Groove diameter is 0.007″ larger or 0.243″ and all fire bullets measuring 0.243″ in diameter.
The various American quarter-bore cartridges such as .25-20 and .25-35 Win., .250 Savage, .257 Roberts, .25-’06 Rem. and .257 Wby have bore diameters of .250 and groove diameters of 0.257″. The .270 Win. has a bore diameter of 0.270″ and groove diameter of 0.277″.
The 6.5mm cartridges such as 6.5×54, 6.5×55, 6.5CM, 6.5 Rem. Mag., .264 Win. Mag., 6.5-300 Wby have a bore diameter of 6.5mm (0.256″) and a groove diameter of 0.264″ or about 6.7mm. Why they have a groove diameter 0.008″ greater than bore diameter, while the larger .270 has groove diameter 0.007″ larger than bore diameter, I don’t know. Most 6.5mm cartridges are European in origin and the Europeans do things their own way. The British back in the day refused to acknowledge the metric system existed. They referred to the 6.5mm Mannlicher as the .256 Mannlicher.
Tikka made the T3 rifle in .223 with different rates of twist and
specified which on the barrel. Dave likes his old Savage 1920
rifle in .250-3000 but the 1-14 twist doesn’t go well with modern bullet design.
Round & Round
Rate of spin is measured, in the U.S. at least, in the number of inches of travel to make one complete revolution. For example most .270 rifles have a rate of one turn in 10″ — but just say 1-10 rate of twist and all rifle enthusiasts know what you mean. Some European metric cartridges have what seem to be oddball twist rates such as 1:8.66. In the metric system, this would be 1:22 or one turn in 22 centimeters.
The biggest change in recent years has been in faster rates of twist. For a long time it was felt best accuracy was achieved with twists “just” fast enough as any imperfections in the bullet are exacerbated by fast twist. Currently there is great interest in longer range shooting and in bullets with high ballistic coefficients. These require faster twists to stabilize.
I can remember when 1-14 was a standard twist rate for cartridge such as .223 Rem., .22-250 and .220 Swift. Today many .223 barrels come with 1-9 twists, 1-8 is not uncommon and some are even faster. There are fads in shooting as in most activities, but this doesn’t feel like a fad, it feels more like the new normal.
