In shooting, like life, there are some things you can’t control. That’s why all your shots don’t go in the same hole, even if you do everything right.

It’s called dispersion, and it’s one of those things a young boy with a rifle just has to learn.

“I had this assumption that I was basically shooting a laser beam and that any influence on the dispersion was from me,” said Seth Swerczek, Hornady® marketing communications manager. “I just thought it was a bad shot and I needed to get better.”

Senior ballistician Jayden Quinlan had the same experience. He recalls competing in a challenge to shoot an egg at 200 yards. He had tested his rifle at 100 yards and got a tight grouping, within the size of an egg, but at the actual event, he missed.

“I learned that dispersion got bigger as a function of range,” he said. “All those years, I felt responsible for the misses. If the bullet doesn’t go in the same hole, it’s my fault.”

Now he knows better. Dispersion occurs at random, and it impacts all kinds of shooting.

“This problem is everywhere, all the time,” Quinlan said. “When I shoot pistols, it’s there. When I shoot a vintage rifle with iron sights, it’s there. When I shoot the field-style PRS rifle, it’s there.”

How does dispersion happen?

Dispersion problems fall into three categories:

• Problems with the bullet
• Interaction between the bullet and the barrel
• Problems with the barrel alone

One potential bullet problem is asymmetric shape. Perhaps the ogive or boattail is crooked.

“There’s probably some element of it that’s always there, but it’s so small at this point with modern manufacturing advancements that it’s not of much concern from a reputable bullet manufacturer,” Quinlan said.

Another issue that can occur during production is center of gravity (CG) offset.

“The jacket material usually starts as a strip, and it’s made into a cup or bowl, and then that gets drawn up into a bullet jacket,” he said. “You might get more material on one side of the bullet than the other, so it’s thinner on one side and thicker on the other.”

That pushes the lead core to one side, creating a situation like an out-of-balance tire.

“If that center of gravity is not aligned with the center shape of the bullet, that means the shape of the bullet kind of wobbles around a bit when it is rotated by the rifling in the barrel,” Quinlan said. Like that bad tire, the wobble increases with speed (velocity and twist rate).

Crooked in the barrel

An example of bullet and barrel interaction that affects dispersion, principal axis tilt (PAT) occurs when a bullet becomes tilted in the barrel. It could happen in the freebore before the bullet engages the rifling, down bore, or at muzzle exit.

Inconsistent bore dimensions created in production or due to fouling can cause tight and loose spots, leading to PAT as well.

“The bullet might start nice and perfectly straight, but halfway down the barrel, the diameter of the bore might start to get bigger allowing the bullet to tip off center in the larger section of the bore,” Quinlan said.

He noted that it’s impossible to make a perfect barrel.

“It’s entirely possible and well documented, actually, that the bore will have curvature relative to the outside profile, or it will not be completely straight,” he said. Like water in a garden hose, the pressure of the burning propellant can try to straighten the barrel.

“If that barrel is in a state of motion, even if it’s imperceptibly small from a vibration or harmonic standpoint, that would increase the level of dispersion if it were variable shot to shot,” Quinlan said.

Misdirection at the muzzle

Problems at the muzzle can happen if one of the lands sticks out farther than the rest.

“There’s a force still contacting the bullet on one side that’s not contacting the bullet on the other side,” he said. “The bullet is being kicked crooked by that last point of contact that is not uniform around the bullet.”

Muzzle breaks, which change the direction of gases to counter recoil and muzzle rise, can also affect dispersion.

“If those gases are not symmetric, meaning there’s more gas on one side of the bullet than there is on the other, you can kick the bullet,” Quinlan said.

“You essentially have nearly infinite levels of clock orientation and combination that a center of gravity offset or tilt orientation can come out in,” he said. “So you start to see the random nature of dispersion.”

“Even when you do everything perfect, these things are at play,” Swerczek said, “but you have some control at the beginning.”

A shooter can minimize dispersion by choosing good components.

“If you get good components — barrel, bullet, powder, the main three — you’re probably going to have a good result,” Quinlan said.

To learn more

To learn more about dispersion, tune into the Hornady Podcast, episode 57.

Hornady® senior ballistician, Jayden Quinlan, also recommends these books:

• Hornady® Handbook of Cartridge Reloading
• “Ammunition Demystified,” by Jeff Siewert
• “Modern Exterior Ballistics,” by Robert L. McCoy
• “Rifle Accuracy Facts,” by Harold R. Vaughn
• “Understanding Firearm Ballistics,” by Robert A. Rinker
• “Ballistics: Theory and Design of Guns and Ammunition,” by Donald E. Carlucci and Sidney S. Jacobson