Article By Gunwerks Staff
At least a couple times a day I answer a question that goes something like this.
“Should I just chronograph my loads and give you the bullet speed and ballistic coefficient?”
I look at my watch and wonder if I should get into it or just say yes to the chronograph. So far I have never said yes to that question, but rather have taken the time to explain a better way.
Over the years we have used several different chronographs, mainly to assist in load development or testing one lot of powder to the next. Some of the newer chronograph units are quite reliable, and we’ve had some good success with the new Lab Radar or MagnetoSpeed. But none have given us enough confidence in true muzzle velocity to rely solely on chronograph data for finalizing ballistic profiles. Some chronographs claim a plus or minus tolerance of 3% on velocity readouts. On a cartridge running 3000 fps this means you could be off 90 feet per second up or down! On one of our 7MM Remington Magnums with a 162 grain Hornady ELDM bullet this velocity variance translates into a 10” – 12” difference at 1000 yards. Throw in an acceptable extreme spread of 15 to 20 fps and you could be off another couple inches at that range. In my opinion the chronograph serves a purpose but it shouldn’t be the end all be all for determining a “true” velocity.
Instead, we use a multi-step process to gather all the right pieces of information to build a turret that will truly match your rifle’s trajectory. It starts with making a drop chart. Once you have a load grouping well in your rifle then it’s time to get out the ole trusty chronograph. Use this to get an approximate speed, a five-shot average will work. Take this chronographed speed to the Gunwerks Ballistic Program. You will need to have an Idea of what the bullet’s ballistic coefficient is as well (you can look at the manufacturer’s website in most cases). We find the most recent published BCs from Berger and Hornady to be pretty reliable. Once you are at the ballistic program page you will find a place to input your chronographed speed, your bullet’s BC, and the environmental conditions where you will be shooting, along with what distance you are zeroed for (we usually do 200 yards for hunting rifles). Calculate your inputs by clicking on the calculate button located top right corner of the page. Then print off your drop chart.
Now the fun starts. Along with your rifle, ammo and other shooting gear take your drop chart to the range. You will need something to read the temperature and you will also need to know your altitude or station pressure, which you can get using a Kestrel, our BR2500 rangefinder, or comparable device. Get set up at the bench and verify your zero at the exact yardage you’re zeroed for. Once verified it’s time to put up a few targets farther out.
I like a mid-range target like 700 yards and a far target around 1000 or so. A half sheet of plywood painted or covered with paper works well, but we do offer some steel targets that are a little more permanent if you plan to be shooting a lot. Now look at your drop chart, see how many MOA or clicks it tells you to dial in for that range and dial it. Adjust your parallax and shoot a five-shot group. Repeat the same with the farther target. Next step, go check out your groups. Chances are you are high or low. If this is the case, with a tape measure, measure each bullet hole from center of bullseye and get an average high or low in inches. Simple, right? You can repeat the process just to make sure. Be sure to take note exactly how much your group center varies from your aiming point as well as the temperature and the altitude when you were shooting.
Step three is even easier. Go back to the Gunwerks Ballistic Program, where you will notice halfway down the page on the right there is a tab labeled “trajectory validation”. Click on it. There are four areas to enter data: load data, environmental data, sighting data, and down range drop data. Enter your load data, which is your bullet BC, bullet weight, and your chronographed speed. Next environmental data, this is the altitude and temperature when you were shooting. It will automatically populate the station pressure and your humidity will default to 50%. Sighting data wants you to input your scope height (center of barrel to center of scope will be adequate), and your zero range. If you are as close as you can be at your zero range but still, for instance, 0.3” low enter this number into the zero-height tab. Use the – sign for low values and + sign for high values. Finally, the last tab, down range drop data. Enter in your far target, the number of clicks, or MOA it took at that range and then if you were high or low after you measured with your tape measure. Enter that value into the zero adjust tab. Click on the calculate tab. Did your velocity change? This new velocity is the true speed based on your drop and the bullet’s ballistic coefficient. Now check your mid-range target drop and your long-range target drop to make sure they jive with each other. If the spread is huge you might need to redo the shooting. If you do, make sure the shooting conditions are in your favor. Looking for down-drafts or up-drafts and ensuring your parallax is adjusted correctly. Eliminating variables from your shooting behavior and shooting conditions will enable you to achieve more consistent results.
Once you are confident in your numbers, it is time to order your turret. The BC you used in these calculations and your new calculated muzzle velocity are the two key bits of info you will need to get it ordered. You will now get a turret that truly matches your rifle’s trajectory, because you validated your chronographed speed. Sometimes the ole chronograph is sufficient, but often enough, it doesn’t get us an exact ballistic profile without a little real-world validation first.
A few years ago, we made a short 15 min YouTube video called, “Validating Your Rifle’s Trajectory”. In that video, you can watch the steps we have outlined here.
—
Editor’s Note:
We’d like to thank Gunwerks for allowing us to re-publish this article. If you’re interested in long-range ballistics, shooting and science the Gunwerks site is an invaluable resource. Find more articles like this at www.gunwerks.com.
