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ARKANSAS SPORTSMAN: Barrel heat hurts accuracy, velocity

by Bryan Hendricks | August 13, 2020 at 2:15 a.m.

A funny thing happened on the shooting range Saturday. I reduced my loads in a troublesome rifle and solved its accuracy problem without losing velocity.

The rifle is a new Ruger Model 77 Hawkeye African chambered in 6.5x55 Swedish. It's a pretty thing, featuring a 24-inch barrel mounted to Ruger's slim, classy and attractive ultralight walnut stock with an ebony forend. It also has a sling swivel mount on the barrel and iron sights. I love Ruger irons, almost as much as I love Remington irons, but the way I've got the rifle set up right now, the irons are purely cosmetic.

As beautiful as this rifle is, it has given me fits trying to get it to shoot. It's not at all like my other Swede, a much older Ruger with a 22-inch barrel. Despite its terrible trigger, that rifle shot a tiny little cloverleaf with factory Remington Core-Lokt ammo right out of the box, and it is one of my most accurate rifles. It's also my favorite hunting rifle, accounting for 13 deer since it entered service in 2008.

With a rifle like that, why do I need a second one? I don't, but I want it.

I had begun to not want it so much after multiple range sessions. I conducted my break-in work and initial accuracy work with factory Hornady Special ammo featuring 140-grain Hornady Interlocks. The rifle did not like that ammo at all, and it shot so inconsistently that I believed that my scope -- a Leupold VX-3 -- was bad. I replaced it with an older Leupold Vari-X II 2-7x36, with equally bad performance.

We're not talking about unusually big groups. We're talking spraying bullets all over the paper.

Having exhausted the Hornady factory ammo, I reloaded the brass with 140-grain Nosler Ballistic Tip bullets on top of 44.5 grains of IMR-4831 powder and Remington Large Rifle Primers. The first two shots with this load couldn't even find the paper.

I moved the target to 50 yards, removed the bolt from the receiver and eyeballed the target through the bore, which allowed me to dial in the crosshairs for a rough zero. Then I backed it up with a laser sighting device to get a little closer. The next three shots printed a .223-inch group, about 6 inches to the left of the bullseye and about 1 inch high.

I moved the target back to 100 yard yards and dialed in the scope to hit dead center and 3 inches high. Eight shots walked progressively up the paper, and I ended the session.

In that rifle, my chronograph clocks that load at a maximum of about 2,670 feet per second. However, I noticed wide variances in muzzle velocity, including a dramatic and progressive loss of velocity as the barrel warmed. Wide speed variances seemed to correspond with wide variances in shot placement.

In all of my reloading manuals, 44.5 grains of IMR-4831 is at or near maximum. I reduced the load to 43.5 grains and took it out Saturday for a short session.

The first shot from a cold barrel clocked at 2,646 fps. It hit the paper dead center and 3 inches high. That's a dead deer shot out to 300 yards. I waited about five minutes before firing the second shot. The barrel was slightly warm, and the chrony clocked the shot at 2,619 fps. It printed the same height and slightly left. I waited another five minutes and fired the third shot. The barrel was warmer, but not hot. The chronograph clocked it at 2,599 fps. It struck near center and 2 inches high.

As the barrel warmed, velocity decreased by about 20 fps per shot.

"If I let the barrel cool back down to room temperature, I'll bet the velocity will go back up," I told myself. "If that happens, I bet I can put a fourth shot through the first hole."

The barrel was cool to the touch until just past the middle, where it got noticeably warmer. That's the pressure point, and the heat continued to the muzzle.

Finally, the barrel reached room temperature. I squeezed off a near perfect shot as far as form was concerned. The chronograph read 2,654. The bullet nearly touched the first hole. Discounting the slowpoke shot, the three shots above 2,600 fps measured .852 inch. The average muzzle velocity was 2,630.

Those four shots taught me a lot about the effect of barrel heat on accuracy and velocity. I had never considered the relationship before.


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