Barrel Heat Hurts Accuracy And Velocity
BY Herschel Smith
3 years, 8 months ago
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.
On August 13, 2020 at 11:01 pm, Bad_Brad said:
That sounds like a nickel alloy. Stainless or chrome moly barrel? I’m betting stainless. 416.
On August 13, 2020 at 11:11 pm, John said:
I’ve fired some rifles including the M14 and old .22’s that had what used to be called
a “wandering zero”. The first several rounds would e dead accurate, then barrel
heat in irregularities in the steel of the would cause it to slightly warp with the temperature
changes acting to move the point of impact in an irregular manner. I believe that
their were processes to correct the problem bu “normalizing” the barrel using
a heat bath and also there was a cyrogenic cooling process involved but it has
been lost to my memory.
On August 13, 2020 at 11:13 pm, John said:
Sorry about some of the mix and match sentences. Old eyes and tiny print don’t go together.
On August 13, 2020 at 11:32 pm, MN Steel said:
I’d change the barrel, or sell it, if I were him.
Firing out of a warm barrel should elevate fps if the cartidge is warmed in the chamber.
Almost sounds like one of the old Mini-14 barrels, my old one would only clover Winchester white box 5.56, and start walking after 6 rounds under a minute.
On August 14, 2020 at 2:34 am, Old Bill said:
Much depends on the powder. So are affected much more by temp. than others. Most think of ambient temps when thinking of powder sensitivity, but it applies to barrel temp also. This is one of the main reasons many love VARGET for 308/7.62; it’s very temp insensitive.
On August 14, 2020 at 9:19 am, MN Steel said:
Old Bill: It says he’s using IMR 4831.
On August 14, 2020 at 5:56 pm, dad29 said:
Bought a Savage bolt .30-06, with scope attached. It was a Savage factory deal.
Discovered the rifle really does not like hot-barrel firing (factory Remington 150 gr.); the rounds marched off the target to the left rather smartly. I dialed, but that didn’t do the trick; then I felt the barrel near the muzzle. Ow ow ow……
Will have to re-barrel-sight the rifle and book about 2 hours’ range time to allow cooling AND re-zeroing at 25, 50, and 100 yards. Ugh.