Twist and Shout
Date: Mar 15 2009
It used to be unless you were a match shooter, you never had to worry much about barrel twist rate. Factory rifles were built using barrels with standard twist rates for the cartridge they were chambered for, and bullet companies made bullets that worked with those twist rates. There have been a few exceptions. When the 6mm Remington was introduced, it had a one-in-12-inch twist. This worked fine for varmint weight bullets but not so well with heavier, longer big game bullets. In 1963 Remington corrected this and started installing barrels with a one-in-9-inch twist.
Nowadays, twist rate has become a critical consideration for just about everyone. This is due in part to the proliferation of longer, all copper bullets and in part because of the demand for bullets with ultra-high ballistic coefficients. Some shooters mistakenly assume the higher the number the faster the twist. Twist rate reflects how many inches it takes a bullet to make one revolution; the shorter the distance, the faster the twist.
For example, Nosler’s 90-grain, 6mm Ballistic Tip with a lead alloy core measures 1.08 inches long. Its 90-grain, 6mm E-Tip – a gilding metal bullet – measures 1.18 inches. According to the Sierra Infinity V6 Ballistic Software’s twist rate calculator, at 3,100 fps, the optimum twist rate for the Ballistic Tip is one in 9.03 inches and one in 7.4 inches for the E-Tip.
Length is why bullets with high ballistic coefficients like Berger VLDs often require a faster twist. A Berger 95-grain, 6mm VLD measures 1.15 inches in length and, according to the Sierra Infinity program, will need a one-in-8.05- inch twist to properly stabilize at 3,100 fps.
My New Ultra Light Arms (NULA) Model 20 .243Winchester has a one-in-10-inch twist, at least that’s what I ordered. I measured the twist by putting a jag with a tight-fitting patch in the barrel, marking the rod and counting how many inches the rod traveled while making one complete turn. I did this three times and came up with three different measurements for an average of 9.93 inches. Close enough? Well, maybe. Measuring twist rate in this manner is not an exact science, but you should do it –just to be almost sure. Just because a barrel says it has a one-in-10-inch twist does not mean the twist is exactly one in 10.
I tried more than a dozen different loads in that rifle, varying seating depth, powder and velocity; and the best group I could get with the E-Tips was just over 3 inches at 100 yards. (For what it’s worth, I tried the Nosler 90-grain E-Tips out of a Sisk custom .243Winchester with a one-in-7-inch twist, and you could cover all three shots with a dime.) With 95-grain VLDs the NULA rifle averaged 1.4 inches. That’s not great, but considering I’m operating with a twist rate 24 percent slower than what is suggested, it’s not too bad. Thing is, other than poor accuracy, there was no indication of improper stabilization; the E-Tips and VLDs were not key holing the target.
The question begging to be asked is how close do you need to be to the optimum twist rate to get good accuracy? Sierra’s Infinity program makes the calculating easy. You just input the bullet weight, caliber, length and velocity, and the program kicks out the optimum twist rate. If you get a twist rate more than 10 percent faster than the twist in your rifle, chances are your barrel will not stabilize that bullet very well. But it might. This is if you are using a stability factor of 1.25 which is what Sierra suggests. Drop the stability factor to 1.0 and the margin of error decreases.