Over the past few weeks, I’ve talked trash about BDC reticles, praised Christmas tree reticles, and explored various zero distances. In all of those posts, I’ve posted various ballistic charts showing how taking things at face value can lead to some significant accuracy errors. As such, I figured it couldn’t hurt to spend a little bit of time talking about velocity and how it relates to accuracy (and precision). Specifically, I want to spend sometime talking about the problem with taking advertised velocity at face value.
Before diving into it, I should explain the difference between accuracy and precision. Accuracy and precision are commonly thought of and used as synonyms. I’ll be the first to admit that I often use them interchangeably without a second thought. However, for the purpose of this discussion it’s important to distinguish between them.
Accuracy, for the purposes of this discussion and in terms of long distance and precision shooting lingo, refers to the projectile being on target. An accurate shot is true and correct. As I write this out, I realize that saying a shot on target is an accurate shot is misleading. It’s also really hard to define accuracy without mentioning precision. In fact, I think the image below that compares and contrasts precision and accuracy does a better job at getting the point across that I can with words. Even so, I’ll give it one more go by saying that a group of accurate shots will leave impacts that are evenly distributed around the bullseye even if the distribution has a large diameter.
Precision on the other hand refers to repeatable mechanical exactness. A group of precise shots will have a distribution with a small diameter. The smaller the diameter of the group the more precise the shots are.
Hopefully, I’ve done an adequate job of explaining the difference between accuracy and precision for the context of this discussion. Even if I haven’t, let’s jump into my beef with advertised velocities.
So what’s my beef?
Before I first tried my hand at long distance shooting, I spent a good deal of time getting the rifle ready with a good scope, selecting a high quality match grade cartridge, configuring the ballistic calculator on my smartphone, and getting familiar with the MRAD adjustments at 100 yard increments along with the corresponding windage adjustments for 5, 10, and 15 MPH crosswinds out to a 1000 yards. I was pretty confident going in with that data and a good 100 yard zero with the rifle and ammo combination. However in my ignorance, the ballistic data I became familiar with used the advertised ballistic data of the cartridge printed on the box. Here is what that data looked like (with only the 10 MPH wind drift adjustment):
|Range (Yards)||Velocity (FPS)||Trajectory (Inches)||Wind Drift (Inches)|
This is all well and good. Or so I thought. The instructor I hired did a good job to make sure I had the proper tools and a basic understanding of long distance shooting before I started slinging lead down range. We started by confirming zero at 100 yards. That went well. Then he put me on a target that had a range of 305 yards. I made my adjustments and hit the target several times. However, the instructor noticed the shot group was centered about an inch above the center of the target. Before continuing on, he stopped me and asked if the shots felt good or if the recoil was bothering me? I was happy with the shots. He then asked if I had used a chronograph to measure the velocity of the cartridge loads I was using? Of course, the answer was no.
As can be expected of a good instructor, he was prepared and brought out his Labradar chronograph and had me shoot a full magazine (5 rounds) to get an average reading. Turns out the average muzzle velocity from my rifle was about 90 FPS faster than the advertised muzzle velocity.
Plugging the measured muzzle velocity into the ballistic calculator gave me a set of new MRAD adjustments to reference. Here is what the updated data looked like.
|Range (Yards)||Velocity (FPS)||Trajectory (Inches)||Wind Drift (Inches)|
At 305 yards, the accuracy of the ballistic data using the advertised velocity was good enough. I was only about an inch high. However, let’s look at the difference between the two data sets to see how inaccurate the shots would have been at longer distances.
|Range (Yards)||Trajectory Difference (Inches)||Wind Drift Difference (Inches)|
The ballistic data difference suggests that using the advertised velocity would have been accurate enough to hit the 10″ targets I was engaging out to about 800 yards. Wind drift really didn’t change much, but the ballistic data using the measured velocity was still more accurate. However, looking at this only from an accuracy perspective doesn’t tell the whole tale.
Consider for a moment that I am able to shoot a consistent 1 MOA group with this rifle and ammunition combination. I can actually shoot a small group with this combination, but a 1 MOA group makes it easier to illustrate how precision comes into play. With a consistent 1 MOA shot group, we can assume all impacts will be distributed in a circular area with a 1″ diameter at a hundred yards. At 300 yards, the impacts are distributed in an area with a 3″ diameter. At 600, the diameter is now 6″. At a 1000 yards, the impact area has a 10″ diameter and completely covers the 10″ target I was engaging. Using the difference between the measured and advertised velocities, I’ve attempted to visualize the shift of the impact area (in black) over a 10″ inch target at 100, 300, 600, and 900 yards in the image below.
What does this image say about the advertised velocity?
I’ll take a stab at that. At 100 yards, with a good zero, it means don’t worry about the advertised velocity. It’s irrelevant. We know for sure the bullet will land the accuracy and precision we expect, because the zero has been confirmed and that’s where the projectile will go.
Further out, say at the 300 yards, the story begins to change. The area of impact will no longer be centered on the bullseye. If the actual velocity is lower than the advertised velocity, the area of impact will be centered below the bullseye. If it’s higher, then the area of impact will be centered above the bullseye. We can expect a shift left or right as well depending on the direction of the wind and the difference in velocity. The level of inaccuracy will depend on the difference between the actual and advertised velocities.
As distance to the target increases so does the inaccuracy. At some point, precision begins to work against us. Shots that would have otherwise been on target will start missing. Eventually, no shots will hit because the entire area of impact will be completely off the target.
What’s my point? I suppose my point is when accuracy accounts knowing the actual average velocity of the projectile leaving the rifle matters. Sure, there are plenty of times when a good zero and slightly inaccurate ballistic data are good enough. The problem is it’s not always clear where the limit of good enough is. Is that good enough actually good enough when the target is a deer (with an 8″ vital zone area) out at 300 yards? Maybe. What about the same deer out at 500 yards?The only way to know with a high level of confidence is to reference good extensive recorded data or a ballistic calculator with accurately measured inputs.