Bullet Runout - How It Affects Accuracy

A simple test of sorting ammo and a shooting session at the range will show how bullet runout can affect the accuracy of your rifles.

Bullet Runout - How It Affects Accuracy
Sorting ammunition by concentricity can help eliminate fliers. Set aside cartridges with poor concentricity for field-position practice.

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.

//content.osgnetworks.tv/rifleshooter/content/photos/Rundown-on-Runout-1.jpg
This group, fired using handloaded 6.5 Creedmoor rounds with 0.006 or more of bullet runout, demonstrates how inconsistent concentricity can result in fliers.

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.

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended Articles

See More Recommendations

Popular Videos

Ruger Launches New American Rifle Predator in 6.5 Grendel

Ruger Launches New American Rifle Predator in 6.5 Grendel

OSG's Lynn Burkhead and Ruger's Matt WIlson kick off SHOT Show 2018 by taking a look at the Ruger Predator.

Gun Clips with Joe Mantegna - BULLPUPS

Gun Clips with Joe Mantegna - BULLPUPS

Joe Mantegna talks about the origins of Bullpups.

Tactical Solutions Introduces New X-Ring Takedown SBR Rifle

Tactical Solutions Introduces New X-Ring Takedown SBR Rifle

Keith Feeley of Tactical Solutions sat down with Michael Bane at SHOT Show 2018 to talk about the new X-Ring Takedown SBR .22LR rifle.

Springfield Armory Saint Victor

Springfield Armory Saint Victor

The SAINT' Victor Rifle delivers a lightweight and agile rifle solution while maintaining effectiveness at extended engagement distances.

See More Popular Videos

Trending Articles

The new Tromix .375 SOCOM offers great performance with less recoil than its .458 brother. MSR

Review: Tromix .375 SOCOM

David Fortier - March 15, 2019

The new Tromix .375 SOCOM offers great performance with less recoil than its .458 brother.

On June 2, 2014, Anschutz announced the start of a subsidiary branch in the United States. Industry

Anschutz Establishes U.S. Branch, Separates from Steyr

RifleShooter Online Staff - June 10, 2014

On June 2, 2014, Anschutz announced the start of a subsidiary branch in the United States.

The author takes stock of rimfire rifles he's known and loved. Ammo

.22 Memory Lane

J. Scott Rupp - January 04, 2019

The author takes stock of rimfire rifles he's known and loved.

You're only as good as your weakest link; heed these to tips to make sure your shooting skills don't hinder your rifle's accuracy potential Shooting Tips

How to Shoot Your Best from a Benchrest

Keith Wood - August 05, 2014

You're only as good as your weakest link; heed these to tips to make sure your shooting skills...

See More Trending Articles

More Shooting Tips

Extreme conditions will effect on your rifle and ammunition; while you can't always do anything about it, it's important to know what's happening. Shooting Tips

How Hunting Conditions and Weather Affect Rifle Performance

Craig Boddington - September 27, 2013

Extreme conditions will effect on your rifle and ammunition; while you can't always do...


Hornady Ammunition introduced a new kind of ballistic calculator based on patent-pending Shooting Tips

Hornady Introduces 4DOF Ballistic Calculator

RifleShooter Online Staff - August 19, 2016

Hornady Ammunition introduced a new kind of ballistic calculator based on patent-pending

Guns, modifications and training tips to help you win the West on your next Cowboy Action Shooting Adventure. Shooting Tips

Shooting Secrets from Top Cowboy Action Competitors

Rick Hacker - April 02, 2012

Guns, modifications and training tips to help you win the West on your next Cowboy Action...

You're only as good as your weakest link; heed these to tips to make sure your shooting skills don't hinder your rifle's accuracy potential Shooting Tips

How to Shoot Your Best from a Benchrest

Keith Wood - August 05, 2014

You're only as good as your weakest link; heed these to tips to make sure your shooting skills...

See More Shooting Tips

GET THE MAGAZINE Subscribe & Save

Digital Now Included!

SUBSCRIBE NOW

Give a Gift   |   Subscriber Services

PREVIEW THIS MONTH'S ISSUE

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Get Digital Access.

All Rifle Shooter subscribers now have digital access to their magazine content. This means you have the option to read your magazine on most popular phones and tablets.

To get started, click the link below to visit mymagnow.com and learn how to access your digital magazine.

Get Digital Access

Not a Subscriber?
Subscribe Now