The Rundown on Runout
May 13, 2019
Being slightly OCD, I’ve long sorted my handloads by bullet concentricity. I select those with bullets seated the straightest for zeroing, hunting and competition and set aside less-straight cartridges for practice from field positions. But does concentricity really make much difference?
Theoretically, it should. When a cartridge fires and propellant gases slam into the base of the bullet, driving it simultaneously out of the case neck and into the rifling, if it comes out crooked because it was seated crooked in the cartridge case it engages the rifling cocked slightly sideways. At that point, the projectile must either swedge to a new shape, maintaining its angle but molding to the rifling, or align itself with the bore. Bullet length, profile and composition determine which occurs.
In theory, the shorter the bullet (and the shorter the bearing surface) the more likely it is to simply reshape and continue at a slightly cocked angle. Heavier bullets and bullet designs with long bearing surfaces should presumably be more likely to straighten out. Again, in theory.
Where I have seen a difference is with bullets composed entirely of copper or gilding metal, and it’s something I’ve documented many times over. Because they are much denser, harder and don’t have that malleable lead core, they seem to resist deformation and as a result are better at aligning with the bore.
Because they’re so good at realigning, mono-metal bullets tend to be the most accurate in chambers with lots of freebore, such as Weatherby magnums. If a bullet has to jump to the rifling—usually about 0.20 inch in the case of the Weatherbys—a tough, self-aligning nature is a real benefit.
As the bullet exits the muzzle, another potential accuracy-robbing element comes into play. Unless the base of that bullet is perfectly square to the axis of the bore, gases begin to escape past one side of the base a tiny fraction of an instant before the rest. As the bullet fully exits, the ejecta jetting out at 5,000 to 8,000 fps applies imbalanced, differing force around the bullet, tipping it slightly. The projectile will recover its equilibrium, but the damage is done. Accuracy is compromised.
Boattail bullet designs—commonly used for extended-range precision shooting—are unfortunately the most susceptible to this. Flat bullet bases not only help align the bullet with the bore, they exit more abruptly, giving the unequal gas forces less time to bump the bullet off-kilter. This is without even factoring the potential bullet distortion—however minuscule—and its offect on flight.
Without a doubt, concentricity—or, rather, the lack of it—has the potential to affect accuracy. But how much? And does it always affect accuracy, or are certain types of cartridges more or less susceptible?
Answering those questions comprehensively is impossible given space constraints, but to get a glimpse into the effect of concentricity, I devised a series of simple tests. I used a favorite 6.5 Creedmoor handload consisting of 142-grain Sierra MatchKings in weight-sorted Hornady brass and a .375 H&H handload built with 250-grain Hornady GMX bullets. And although this is a handloading column, for research and comparison’s sake I included a favorite factory load: Federal Premium’s 200-grain EDGE TLR bullet in .300 Win. Mag.
I sorted each type using a Hornady Lock-N-Load concentricity tool, creating two separate 10-round batches. One batch had maximum detectable runout, the other had minimum runout.
Just how much was that? In the case of my 6.5 Creedmoor handload, by sorting through 100 cartridges I managed to separate 10 with .006 to .007 inch of runout. Most had less than .002, and I managed to find 10 cartridges with less than .001. That’s a six-times difference between batches. My .375 H&H handloads ranged from about .002 to .006.
Federal’s factory load demonstrated more variation. From three boxes, I managed to sort out 10 cartridges with about .008 to .010 runout and 10 cartridges with less than .002. This load tends to be quite accurate, averaging less than one m.o.a. in my favorite .300 Win. Mag., but it does occasionally send a flier a half-inch or so wide. Sorting the bullets made me wonder—and hope—that the less-concentric cartridges were the culprit. If so, by sorting the factory ammo I could eliminate the fliers.
As an aside, factory ammo can actually be very consistent. I attempted to sort Nosler’s 55-grain Ballistic Tip Varmint ammo in 22 Nosler and couldn’t find a single cartridge with more than .003 runout, and most were less than .002. A batch of Barnes VOR-TX Long Range .300 Win. Mag. ammo with 190-gr. LRX bullets yielded similar results.
For consistency’s sake, I fired all four rifles from bipods, using a bunny-ear sandbag beneath the toe of the stock. Using a four-spot target, I fired two consecutive five-shot groups with the most-concentric ammunition batch, then two more with the less- concentric ammo.
To give them a fair shot, I allowed the .300 Win. Mag. and .375 H&H rifles to cool between each five-shot group. However, I cooled the 6.5 Creedmoor—a superbly accurate Tikka T3x TAC A1—only every 10 shots.
Interestingly, there wasn’t a smidgen of difference in the .375 H&H handloads. It’s a small sample size, of course, but this could indicate that big-bore cartridges are not particularly sensitive to concentricity inconsistencies, as long as those issues aren’t dramatic.
In each case the less-concentric 6.5 Creedmoor cartridges put four of the five shots into a tiny group but flung one flier . The first group was 0.51 inch, with four shots in 0.28. The second group was 0.71 inch, with four in 0.30.
Neither of the extremely concentric batches produced discernible fliers. Both put all five shots into less than one-half m.o.a., specifically 0.49 and 0.47 inch.
Similar results came with the Federal factory ammunition. The first and second five-shot groups with the less-concentric ammo measured 1.48 inches with four shots in 0.81, and 1.32 inches with four in 0.60 respectively. With the tight-concentric batch, neither group had a flier, and the two groups measured 0.91 and 0.83 inch.
I found the results of my tests conclusive but also perplexing. Clearly, sorting for tight concentricity helps eliminate fliers. According to these simple tests, it appears 80 percent of bullets exhibiting poor concentricity still group well, with only about one out of five actually resulting in a flier. But that’s enough for me to keep sorting my ammo for concentricity.