Does Barrel Temperature Affect Accuracy?

During one of my recent .22 LR ammo tests, I ran into something I wasn't expecting.

It was a hot summer afternoon with temperatures around 32°C (90 F) and the shooting bench in the sun. I was testing Aguila Super Extra 40 gr. through my Henry .22 LR when I noticed my groups changing in a way I hadn't seen before.

That got me wondering how much barrel temperature could be affecting accuracy.

The First Two Groups

I started at 25 yards and fired my first 10-shot group.

The first two shots fired high but then it started to pattern nicely, centered a touch high on the bullseye.

I let my son take a turn with his BB gun for a few minutes before shooting again, with the rifle placed on the shooting bench, in the sun.

The second 10-shot group was actually tighter, but it caught my attention.

The group started on center but continued shifting to the left. 

At the time I wasn't sure what to think but had a feeling the barrel temperature was affecting accuracy.

Moving Back to 50 Yards

Since the ammunition was performing well enough, I moved my targets back to 50 yards.

I parked the rifle in the shade while I set up the new targets and gave it some extra time to cool down.

The next 10-shot group looked normal for my Henry at 50 yards. In my experience, asking this rifle to consistently shoot tight groups at that distance is pushing its limits with most ammunition.

At this point, the barrel was noticeably warm but not overly hot. Curious to see whether it would affect the next group, I immediately reloaded and fired another 10 rounds.

This time the results were dramatically different.

The first shot landed roughly 1½ inches higher than the previous group.

Each shot after that continued drifting lower and to the left, producing a long diagonal string across the target instead of a normal group.

By the end of those 20 rounds, the barrel was hot to the touch.

What Happened?

As a barrel heats up, it expands slightly and its vibration characteristics change. Those changes influence where the muzzle is pointing when each bullet exits the barrel, causing the point of impact to shift or groups to open up.

In this case, the combination of repeated shooting, a black barrel and direct sunlight on a hot day kept the barrel much warmer between groups than it normally would have been.

I'll Be Testing It Again

After turning my ammo test into a barrel temperature test, it looks like I'll have to head back out and do some more shooting to complete the Aguila ammo test.

Sometimes the most interesting part of a range day isn't the planned test—it's the unexpected questions that teach you something new.