Truing ballistics data is something everybody should be doing regardless of skill or experience level. The practice of truing your ballistic data is a fundamental necessity if you want to have accurate adjustments and drop charts for long distance shooting. What we are talking about is taking the data that is output by a ballistic solver or computer and tweaking it to match what we see in real life. You should never just generate a drop chart and assume it will be correct. There are too many variables in play here. You need to generate a drop chart and then check its accuracy with live fire. Then you need to adjust the chart to match what you are seeing in the real world.
How to go about Truing Ballistics Data
Generally there are two major variables that will give you errors when comparing live fire results to computer generated ballistic data. Those variables are ballistic coefficient and muzzle velocity. Obviously if the ballistic coefficient of your bullet is higher or lower by an appreciable margin it will perform differently than what your ballistics engine is telling you. Similarly, if the bullet is moving faster or slower than you think it is, you will see errors when comparing generated ballistic data to what it takes to hit a target in real life.
The first thing you should be checking is your muzzle velocity. Particularly if you are using a traditional light sensor chronograph to derive the speed of your projectiles. If a bullet is moving faster or slower than you think it will require a different amount of elevation and windage to hit your desired targets at any given range.
Since traditional light based chronographs are somewhat prone to error, the odds are that if you are seeing large discrepancies between your data and live fire, the error is in your muzzle velocity value.
If everything matches up with regard to your muzzle velocity but you still are getting large differences between your generated chart and live fire results it could be that the ballistic coefficient of the bullet is incorrect. Most times the error here will be garbage in, garbage out. If you plug the wrong BC into your ballistic computer or fat finger the value that will show up as an error when you go to shoot. If everything has been plugged in correctly you should consider the possibility that the published BC of the bullet is incorrect. Typically when they are wrong, they are inflated values. This would manifest itself with live fire that needs more elevation or windage to get on target at a given range than what the computer says you need.
When we discuss BC and MV as the likely culprits to bad ballistic output, we’re assuming you’ve done everything else correctly. Rather than make too large an assumption I’m going to mention a few other variables that could be causing your problems. For starters, your zero. Check it if your adjustments seem like they are off. I shot a whole afternoon one time thinking my data was bad and I had lost my touch until I checked my zero and it was off by a considerable margin. Sometimes you bump the gun or the dog bangs into it, whatever, shit happens in life so definitely check your zero.
Have you measured your height of the optics over the bore? That should be built into your ballistic computer somewhere. Make sure you measure from the center of the scope bell to the center of the barrel. This isn’t something that will garble your ballistic data alone but if combined with other errors they can all add up pretty quickly. Double check you have this measured correctly and included in your ballistic settings somewhere.
Are the screws all tight? If your base is loose on the action, or the scope rings are moving on the base, or if your scope ring caps are loose you will have issues. Make sure your action is tight in the stock. Just go over the whole rifle with a torque wrench and some bits and make sure everything is tight and torqued properly to the appropriate specification.
How sure of the range to target are you? It may be time to upgrade your rangefinder or to double check that the firing line is really at the distance your shooting club says it is. If you have poor range to target data you will obviously have issues with your dope and making hits. Verify the range to target with a friend’s laser if you aren’t positive of the distance. If you don’t have one try using a GPS app on your phone to compute the distance between two sets of coordinates. Pace it off if you have to but don’t take for granted that a firing line says ‘800’ and believe it really is. Here’s another thing to check, is the range measured in meters or yards? Don’t look at an 800 sign and assume its yards when it may be meters. Things to check!
Don’t neglect weather data. Check your environmental variables at the shooting position with a Kestrel. You can get used ones pretty cheap, its well worth the investment. If you use a weather station for your environmental data you are using data from a weather station that can be several miles away and the same data may not be true for your shooting position. Check your weather data at the firing position I strongly suggest you avoid using weather station data or phone apps to get this information for your ballistics program.
Truing Ballistics Data
If everything is as it should be and you still have issues, try truing your data based on muzzle velocity first. Shoot at the greatest distance you can, the farther the better, and note the adjustments necessary to get on target at that distance. Then halve that distance and note the adjustments needed to get on target at that range. Now pull up your ballistic program and start changing the muzzle velocity. Phone apps like the Applied Ballistics application actually have a tool for doing exactly what we’re describing. You can plug in the distance and the adjustments and it will computer the velocity necessary to register a hit with the information you have plugged in.
If you don’t have a neat tool like that you can do it old school and just fiddle with the muzzle velocity in 50 and 100fps increments till you get closer and narrow it down. The reason I suggested halving the farthest distance you have access to is to double check the new chart. If you shoot at 800 yards and the chart matches when you make your muzzle velocity 2700fps that’s great. However, you need to make sure its reasonably close at 400 yards too. If its dead on at distance and way off at the halfway mark there is something else going on.
That something else might be ballistic coefficient. Try tweaking that a little bit at a time, small changes make big differences. Start with the farther distance again and then check the accuracy closer in and see if things line up. If they do, that’s great! You solved the problem. If they don’t you may need to look at some of the other factors we mentioned above. If you are still having trouble, email us or leave a question in the comments and we’ll try and get you sorted out!
Muzzle velocity and ballistic coefficient are the two main culprits when dealing with discrepancies between generated charts and live fire. However, we assume you have the rest of the variables taken care of when making that statement. If you aren’t sure, go back over the rifle and all the variables and make sure they are correct. Then if all is as it should be you can start tweaking your muzzle velocity and your ballistic coefficient to get your live fire data to match your computer generated ballistics chart. Remember the published BC numbers of projectiles are often fairly optimistic, in real life they may be slightly less impressive. That’s okay, but it needs to be built into the computer that is doing your ballistic calculations!