March 01, 2011
One afternoon on a cold winter day about 20 years ago, I missed a 300-yard broadside shot at a whitetail doe on my own home ground.
Let me explain why it upset me so much.
I was serious about getting a deer. It was the last hour of that year's deer season. I'd gone scoreless so far, we needed meat, and I was shooting a rifle that I knew was accurate. It was a solid setup from a steady tree stand rest. I fired three times. The doe apparently had no idea there were bullets anywhere close. So I quit wasting ammo and went home empty-handed.
I miss with the same embarrassing frequency as any other hunter. But I usually know why. This time I didn't. The particular rifle was a Howa Model 1500 .308 Winchester with a natural-finish oiled-walnut stock, 24-inch barrel, high-end 6-24X scope and proven match-accurate Hornady ammo. I'd verified the zero before hunting, and I knew that setup's trajectory out to 300-350 yards as well as I knew anything. But I missed so bad the deer didn't even notice.
Did I mention it was about 10 degrees below zero that afternoon?
I had been sitting out in the icy cold, morning and afternoon, for three days. I had gone out before daylight the last day and sat until late morning, then took lunch and a warm-up break -- leaving the rifle in the stand under a cloudless sky -- and returned for the final three hours of waiting. It was bitter, and the chill factor was fairly ominous.
Then, just at the crack of dusk, as the wind died, came that doe. Three shots later, without even any indication of flying debris anywhere that could indicate where I might be missing, I simply sat the rifle down and watched while she ate her way across the bean-stubble ridge and out of sight. After a while, I got down from the tree and went home.
I carefully wiped the rifle down when I got home -- letting it warm up slowly to avoid a lot of condensation moisture -- and set it in the rack without fiddling with the scope setting or any of the stock screws. About two weekends later, when the weather provided a moderate day, I took it out to the range to check it out. At 200 yards, it was still zeroed and at 300 yards had its predictable 10-inch drop. I was not happy. I grabbed some paper targets and a backing board, got in the Jeep, and drove to my tree stand, where I set up the targets at the spot the deer was standing, went up into the stand, and benchrested several groups from there. Bullet impact was right where it should have been -- about 10 inches below the crosshair intersection. So why, tell me, didn't I hit that deer?
I can hear you saying: "Because of the cold, stupid."
It took me way too long to figure that out, but I was dumber 20 years ago than I am now. And, frankly, I didn't expect the "cold-weather effect" to be as much as it was. Most of us know that extreme cold (or heat) has an effect on the burn rates of different types of ammunition propellants, which has a consequent effect on bullet velocity and trajectory. And there are charts available that show this for certain propellants, as well as the trajectory profiles available for bullets at different velocities. But, generally speaking, if you apply such charts and data to the situation I encountered, you only get about a two-inch maximum impact displacement with a temperature difference between zero degrees and a "normal" 70-degree day. Not enough to explain how badly I was missing.
How bad was that, actually?
Most temperature-effect charts consider only the propellant powder temperature issue. But cold has other effects on a gun as well. When it is so bitter outside that you can hear trees creaking as the moisture inside them swells and contracts, what subtle shot-displacing effects may be taking place in the wooden stock and metal action of your rifle? Taking it from your living-room gun cabinet to a sub-zero chill factor should certainly be suspected to have some persuasive effect. So, the only way I could think of to find out about actual temperature effects on the .308 cartridge I was using in my rifle that day was to actually fire it under controlled conditions at different temperatures.
The warm part was easy. During September a year after the incident, I went out one afternoon when the temperature was a seasonable 75 degrees and verified a point-of-aim zero at 200 yards. Drop at 300 yards for an average of five 3-shot groups was 10.5 inches. Then I put the rifle away and waited. During the first week of January, the weather finally obliged by dropping into a period of subzero nights. I took the gun one afternoon to my indoor firing bench, where I can sit in comfort and fire out through a slightly raised window, and verified that the established point-of-aim zero and 300-yard drop was still good at "room temperature." Then I loaded its magazine, and set it and several boxes of ammo outdoors for the night. The overnight low was minus-6 degrees. I was at the outdoor range at 7 a.m. the next morning and started firing 300-yard groups when the temperature was right at the zero mark.
The target paper was a 30-inch square, with a target dot 10 inches down from the top for an aiming point. The first three shots didn't print on the paper. I shifted aim higher, then lower, then sideways, and finally dropped back to a 100 yard target to find where things were going. Way low and way to the left. Then I reshifted to 300 yards and determined that the actual point of impact was about 45 inches -- nearly four feet! -- to the left and in front of the target board. No wonder that doe wasn't bothered by my shots.
Afterward, I took the rifle back home, let it come back to room temperature by sitting in the rack for a couple days, then went back to the "open window" range and tried it again with room-temperature ammo. It was basically back at its original zero point, offset maybe two inches from the original setting.
The whole episode opened more doors than it closed. I would never have believed that there could be as much as a 45-inch difference in point of impact at 300 yards caused by an ordinary autumn-to-winter outdoor temperature difference. But there it was. It's obvious that temperature differences, particularly large seasonal changes, do make a measurable difference in high-power rifle/cartridge performance. The amount of that change varies widely from gun to gun and from cartridge to cartridge. My wood-stocked Howa .308, which did not have a free-float barrel, was pushed substantially off true by its stock's reaction to extreme temperature (and humidity) changes. Overall, it also definitely appears that synthetic-stocked rifles are more resistant to temperature changes than wood-stocked ones, and that natural-finish, oiled-wood stocks are probably the least resistant. When winter -- or summer -- comes, a wise rifleman should check to see what his rifle and load combination is going to do at the temperature the field provides, before depending on them to put meat on his family's table.