Barrel Heat Hurts Accuracy And Velocity
BY Herschel Smith
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.
I hadn’t either. Thanks for sharing this in such detail Bryan. No doubt I’ll put this information to use.