Ever see those little charts with adjustments on them that tell a rifle shooter how many clicks of adjustment they need to hit a target at a specific distance? It’s called a drop chart and it’s the subject of this post. A lot of people don’t know where to go or how to generate one so we’re going to tackle that and explain it all. This way all the guys that buy a rifle and want to reach out a bit will have a guide for how to generate a drop chart that will enable them to stretch the legs of their rifle without any of the guesswork that can be wasteful of ammunition and provide a lousy shooting experience!
What is a Drop Chart?
A drop chart is a chart that shows the adjustments needed on a rifle scope to move the reticle in order to account for the pull of gravity on the bullet between the shooter and the target. Essentially it’s giving you a figure that you will dial in that will alter the angle of the scope in relationship to the angle of the bullet leaving the muzzle. The purpose of the change is to allow you to lob the bullet out to the target. The bullet will begin to fall towards the ground as soon as it leaves the muzzle, so it stands to reason you have to shoot at an angle to give the bullet the extra loft it needs to reach the target. You don’t throw a football straight to your buddy, you have to lob it, same principle.
That is an example of a drop chart. The first column shows the range in yards. The second column shows how far the bullet falls in inches at that range. Next up is the drop adjustment needed to account for the fall of gravity in MILs. Then it starts over with the windage in inches and windage in MILs. It also shows the speed, muzzle energy, time of flight, and lead amounts for a moving target at the same range. Unless you plan to encounter a moving target, the drop and windage adjustments in MILs or MOA are what most people will paste into a spreadsheet and print out as a chart. You can then tape that to the side of your rifle for easy reference when hunting or at the range for the day.
Generating a Drop Chart
I’m once again going to suggest, especially for newer shooters, the use of the free ballistics engine available at JBM Ballistics! Once at the JBM site click on the calculators button and then either the Trajectory or Trajectory-Drift calculators. The difference between them is the Trajectory-Drift calculator will take the effect of Spin Drift into account while the former will not before it makes a ballistic prediction table for the shooter. For this article we’re going to use the Trajectory calculator to keep things simple for the newer shooters. We can get into more advanced ballistics tables later.
Now that you are using the Trajectory calculator there is information you have to plug in before you can get the chart printed up. Garbage in, garbage out, so try to use the best information you have because goofing the inputs will give you wonky trajectory charts that won’t be as accurate. JBM has an excellent bullet library that you can use marked Library. Select the bullet that your rifle is firing from the list and the library will prefill the information about the projectile, like it’s length, weight, and ballistic coefficient into the calculator for you. I suggest you do that if possible to keep things simple. Let’s use a 175gr Sierra HPBT in 308 Caliber.
The next field you need to be concerned with is muzzle velocity, how fast the bullet is moving. This is where the use of a chronograph becomes extremely handy. If you don’t have a chronograph and can’t tell what your muzzle velocity is, you can look at the box of ammunition and it will typically have some velocities printed on the box to get you in the right ballpark. The less accurate the muzzle velocity, the less accurate the chart, so be mindful of using velocities on the box. It should get you close but as distance increases accuracy is going to fall off.
When finished with muzzle velocity enter the distance to chronograph if you are using one, if using a printed velocity from the box enter zero. Next plug in your sight height, how high the center of the scope is above the center of the barrel. Leave the wind speed settings at 10mph from 0900hrs. We explained why in our article on Proper Wind Call Communication! Pressure, temperature, humidity, and altitude are up next. Make sure you use pressure measured with a Kestrel if possible, and uncheck the box that says ‘pressure is corrected’ which means corrected for altitude. If you don’t have a Kestrel use the pressure from your local weather station and leave the box ‘pressure is corrected’ checked.
The range options should make sense for minimum and maximum range and how big you want the steps or increments to be. Temperature, humidity, and altitude are fairly straightforward. Plug those in, numbers from a Kestrel are best because they are conditions that represent where you are shooting and not where a weather station is reporting from that may be several miles away. Do the best you can and you should be okay. The last box is ‘Column 2 Units’ change that to MOA or MILs so that it matches the adjustments of your scope. That way you will get a column of adjustments tailored specifically to the click values of your scope and calibrated for the ammunition you are firing from your rifle.
Wrapping Up Drop Charts
You absolutely need to learn how to do this so if you haven’t done it before, start goofing around with JBM and how to work up some ballistic drop charts for your rifle and ammunition. This is one of those things that once you do it enough it will become second nature and you’ll be able to enter the variables from memory, generate the chart, and check it’s accuracy against your recorded results from live fire. Remember if the chart doesn’t match what you see in real life, the chart is wrong. The bullet gets the final say so something in your settings is off, the conditions don’t match, muzzle velocity isn’t correct, etc. As you gain experience you will get better at diagnosing which variable is the likely culprit. That may make a good article of its own in the future!