Why Different Rifles Prefer Different Ammunition

With my recent ammo testing, I started thinking - How can different brands of ammunition shoot so differently from the same rifle?

Certain rounds consistently produced tight groups, while others could hardly produce a pattern of any kind.

Sometimes even high quality, well-respected ammunition would shoot poorly from a particular rifle.

I found myself saying "I guess this gun doesn't like this ammo". 

But then I started wondering why.

Every Rifle Is Slightly Different

Even two rifles of the same make and model are not perfectly identical.

Small differences in the chamber, barrel, crown, bedding, trigger, and manufacturing tolerances all influence how a round interacts with a rifle.

Those differences may be measured in thousandths of an inch, but by the time a bullet reaches the target they can translate into big differences on paper.

Ammunition Is More Than Bullet Weight

During my 22 WMR ammo test with my CZ 457, I was unable to produce any respectable groups with 50 gr. ammo.

After looking into it, I learnt that while bullet weight matters, it's only one part of the equation.

Changing bullet weight changes several other characteristics at the same time, including bullet length, velocity, pressure, and the amount of time the bullet spends traveling through the barrel.

Changes in any of these variables can affect how the bullet travels through the barrel, exits the muzzle, stabilizes and flies toward the target.

Consistency Matters

One of the biggest differences between premium and bulk ammunition is consistency.

No two cartridges are perfectly identical, but higher-quality ammunition is generally manufactured with tighter tolerances.

More consistent powder charges, bullet dimensions, lubrication, and ignition all contribute to more repeatable performance.

These differences are most obvious when shooting 22 LR. For those that enjoy shooting bulk ammo, sometimes you'll notice a shot that immediately sounds wrong. When I'm shooting on paper, this almost always results in a shot impacting away from the rest of the group. 

With higher quality ammunition, you increase the odds of every shot behaving similarly, which results in a better chance of producing tight groups.

Barrel Harmonics Play a Role

Each shot behaving similarly becomes more important due to barrel harmonics. 

When a rifle is fired, the barrel vibrates as the bullet travels toward the muzzle.

This vibration exists for all rifles, including rimfire.

Ideally, each bullet exits the barrel at nearly the same point in that vibration cycle.

Inconsistencies in ammunition cause the bullet to exit at a slightly different point during that vibration. Those tiny differences become larger groups downrange.

This helps to explain part of why that dud of a 22 LR round becomes a flyer. 

Barrel harmonics are part of the reason why hunters stick with a certain type of ammo once a rifle is sighted in, having witnessed other rounds impacting differently despite having similar bullet weights, velocities, etc.

Every Rifle Has Preferences

I found this with my Henry .22 LR.

After testing multiple brands at 50 yards, I was ready to admit the rifle isn't capable at this range. For no good reason, I had a box of 42 gr. semi-auto ammo and gave them a try. 9/10 rounds formed a 1/2" group - better than I expected from a lever action 22.

It's difficult to nail down what exactly is causing a rifle to shoot certain ammunition well while it struggles with others. Typically it's multiple factors combined.

This is why it's important to test several different types of ammunition to get a feel for what the rifle prefers.

Sometimes the results are exactly what you'd expect - pay a bit more for ammo, get a tighter group.

But it's not always that simple. Sometimes the right round is the one you least expect.