The weather was especially nice for early January, 50° but windy, and I needed to get out to the range. I wanted to shoot my scoped AR. I’ve been reloading 68 grain JHP in .223 Rem for it and I wanted to make a joyous noise. I also wanted to try out another project.
Since the sniper match last year (http://tactical-talk.blogspot.com/2013/10/ipsc-sniper-or-shoot-faster-faster-you.html) I’ve been wanting to see how my point of impact changes when the rifle is rotated from its normal shooting position. You know, bore squarely under the scope and both perpendicular to the center of the earth. When the bore and sights aren’t in this orientation bad things happen.
Bad things? You usually miss the target.
|Sometimes you have to incline the rifle to shoot through an opening. This creates problems. Note bubble level on scope.|
The blog, “Things Worth Believing In” has an excellent overview of rifle twist and how your impact changes when the bore isn’t directly under the sights. Check Tom’s blog “Twist and Shoot” for more information. Keep reading to "When things Go Sideways." I wish I had thought of that title.....
Here’s my take on this.
Since Galileo we have known that an unsupported object starts to drop the instant support is removed. The instant the bullet exits the gun barrel it too starts to fall. To get the bullet to a target farther down range than dozen feet or so we incline the barrel upward so some of the force used to push it down the barrel is also used to push it upward. At some time during the projectile’s short life, gravity saps all the upward motion and it starts to fall. Almost any explanation of external ballistics will show you images like:
Both cartoons show how the rifle barrel is inclined so the fired shot crosses the line of sight. The rifle is set up this way to give the bullet some upward force to resist the pull of gravity. Eventually gravity pulls the bullet downward and back across the line of sight.
So what happens when I twist the rifle so the bore and sights aren’t one under the other and perpendicular to the center of the earth? Doesn’t the impact just shift by a small amount?
Several things happen. One, the barrel isn’t aimed at the same point as your sights. How much difference will depend on the amount of displacement from normal position, the distance between your sights and the bore and, I suspect, your zero.
What complicates this is we forget that the bullet no longer has that arc upward to help it resist gravity so it starts to fall as soon as it leaves the muzzle. And it falls at 32 ft per second squared or 32 feet in just the first second.
The difference between your impact and your aimed point of impact will change depending on the distance as well as the amount of offset. Have your rifle twisted sideways a little and you’re off a little at 25 yards. Shoot at an elk at 300 yards and its likely all you do is frighten the animal and give it some exercise.
I had previously zeroed my scoped rifle at 100 yards on a nice dry 80° day. Today was 48° (and now rainy) so I expected some change in performance off sand bags and I got it.
|I'm set up at my bench. The equipment is simple: a sand bag, ammo can to hold the sand bag and my scoped rifle.|
|I like using these targets for rifle 'cause there's five of them and each has a letter or number so I can take better notes on my progress. This is 100 yards from the bench.|
I set up my shooting station and put my target out at 100 yards.
My scope uses a chevron as the aiming point and placing the black chevron on the center of the back target was difficult. I had a certain amount of uncertainty holding a black chevron on the center of a black target.
|Low and at the 6-7 divider. Not very good off a sand bag.|
When I inspected the target it looked like my zero had drifted. Then it occurred to me, I could illuminate my chevron so it stood out on the black target.
So I turned on my light and re-shot my zero. I was much happier with those results. Considering the wind, the temperature and shooting with gloves on, I’m happy with that group.
I then turned the rifle so the scope was between 10 and 11 o’clock and the magazine between 3 and 4 o’clock. (Yes I know it’s redundant to describe both positions, but I feel the need to be well defined in this experiment). I re-positioned my rifle so the chevron was centered on the center and shot my group.
I’m sure the fellow shooting at the other end of the range must have thought I was nuts and I’m sure that was confirmed when I went down range and photographed my targets.
How did my group come out? Well, if I was aiming at a man-size target to hit the heart, I would have caught his lower right lung. I was 5 inches low at 7 o’clock. That means I need to shift my aim up to 1 o’clock about 5 inches from the center to drop the rounds into the center black.
|I've offset the relation between rifle bore and rifle sight by 45 degrees. Big change in impact!|
I’ve been thinking of constructing an exercise for the monthly rifle group meeting this spring to demonstrate this problem and give them a chance to practice and adjust for this shift of impact. But I’ve got to tell you, it’s difficult to shoot a rifle twisted 45 degrees off a sand bag rest from the seated position. The butt stock barely contacts your shoulder. If the caliber was any bigger than .223 you might have real trouble controlling recoil.
My thought experiment from this test suggests canted rifle shots will be low and displaced towards the target side opposite of the rifle’s magazine. My thought experiment does not describe what happens at close range (between the muzzle and your zero) or how the displacement increases past your zero.
It does point out why some hunting rifles have a bubble level on the scope and bi-pods have independently adjustable legs.