Shot Variance: What Went Wrong?
by Glen Stilson
Here in Arizona it’s no small feat to get drawn for a good hunt in our big game tag lottery. A rifle deer tag may only happen once every few years, sometimes longer, and once drawn there are not many hunters that I know who don’t put their heart and soul into the hunt, including pre-hunt scouting and equipment preparation. What a disappointment it is when they finally get to the goal line only to miss the shot on that big beautiful buck.
The question is often “What went wrong?” The answer is usually something to do with the shooting fundamentals (i.e. jerking the trigger or shot anticipation), but not always. Let’s take a look at a few of the things that many hunters do not consider when they are rifle hunting.
Clean Bore and Cold Bore
A clean barrel and a dirty barrel do not have the same point of impact, so it’s important to head into the field with a dirty barrel – not to mention the smells that can come from various gun cleaners and solvents if you clean your gun right before a hunt. Cold bore is the variance in point of impact on the very first shot out of the barrel, and this should be recorded on every range trip so that you can figure out what it is. Cold bore may vary anywhere from 1/2” to 2” (or more) from zero at 100 yards, and when you add in clean bore variance, this can really affect your first shot, which may be your only shot.
For every 10 degrees of air temperature change from the temperature that you zeroed your rifle in, count on a 1/2” or .5MOA point of impact shift at 100 yards. This impact will be up in elevation if the temperature is hotter than your zero temp, or down in elevation if the temperature is cooler. As an example, if you zero at 80 degrees and then head to the mountains to hunt where it’s 50 degrees, expect to see a minimum of a 1.5” shift down in your point of impact at 100 yards.
I have seen a lot of spare ammunition holders strapped to the sides of hunting rifles, and while this may make a load or a reload much faster, ammunition temperature has to be considered. If the rounds are on the side of the rifle baking in the sun, or freezing in the cold wind, it can affect where the round will impact when fired. You will typically see a 1” or 1MOA shift at 100 yards for every 30 degree change, and just like ambient temp outlined above, hot means the point of impact shifts up, and cold means the point of impact shifts down. If you have your ammunition on the side of the rifle and it’s been baking in the sun all day, and the temperature of the rounds have reached 90 degrees while your zero temperature was 60 degrees, then you can expect to see a point of impact that is at least 1” higher at 100 yards than what your zero is.
One of my past articles for Western Whitetail was titled ‘Positioning for a Clean Kill’ and covered several effective field shooting positions, so I won’t go into that here, but I do want to discuss the effect of pressure and tension in shooting positions. It’s very common to see a shooter really put a lot of pressure on their rifles when practicing shots, because they are mistaking pressure for leverage. If you have a great amount of tension and/or pressure trying to hold the rifle in place for a good shot, or due to shot anticipation, then when you fire the round that tension and pressure will typically release, causing a shift in the barrel direction and sight alignment, which will change your point of impact. It’s important to practice from field shooting positions where you can be comfortable and sit for a few minutes if needed, without causing you fatigue or the shakes.
How It All Comes Together
So how does this all affect you? Let’s set up a realistic scenario. You zero your rifle at the shooting range at 100 yards from a comfortable bench position. It’s 75 degrees outside and you lay your ammunition out on the table so that it’s easily accessible. You fire your first 3 shots and adjust your zero based on the average group size. A few more rounds to confirm zero and you’re set. You go home, clean the barrel and grease the bolt, and put the rifle back in the safe for this weekend’s hunt.
Now you’re out in the field, and it’s 45 degrees with a cold wind, but you’re dead set on pursuing that trophy buck that you’ve been scouting for the last week. As you approach the area where you expect him to be, you load your rifle with the rounds that you keep in that handy buttstock shell holder. You make your stalk to the meadow and wait, but it doesn’t take long before that big buck steps into view. He’s 200 yards away and quartering towards you, and you balance your rifle on the nearest tree limb, working hard to keep the stock firmly pressed into your shoulder. You line up the scope crosshairs, press the trigger smoothly to the rear, and BOOM! The rifle fires, but the buck darts away into the brush. You rack another round into the chamber but can’t find the buck in the crosshairs. What went wrong? Let’s look at the math for a shot fired at 200 yards, which means that we’ll double all of the variances listed above due to the shot being twice the distance as your zero:
- 30 degree shift in ambient temperature = 3” shift down
- Minimum 30 degree shift in ammo temperature = 2” shift down
- Cold bore variance = varies based on gun and ammo, but let’s go with a shift of 2” to the right
- Clean bore variance = again based on gun and ammo, but let’s add another 1” right
- Position = the pressure of the shooting position, plus a little ‘buck fever’, can add in all kinds of shift in point of impact, but for this example let’s say you only affected point of impact by 4” down, not an uncommon shift (2MOA) or an uncommon direction for that type of mistake
So when we add it all up we come up with an impact that is 9” low and 3” right. Even with no mistake in the shooting position and trigger press, the shift would have still been 5” low and 3” right, which depending on where you were holding your sights and which direction the animal was facing, can result in a missed shot at worst or a wounded game animal at best, which decreases your chances at success.
My best advice is this: in addition to practicing more and seeing just how much these variances can affect your shots (and recording the results!), it’s also a good idea to confirm your zero and adjust your sight(s) accordingly at the location and in the conditions where you will be hunting. It’s always better to be standing over that cleanly killed trophy smiling at what went right than asking yourself “What went wrong?”