truing-ballistics-data5

Truing Ballistics Data

In Blog by Rich6 Comments

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.

truing-ballistics-data1

In red is the ballistic coefficient of the projectile, you can play with this number if necessary, in blue is the muzzle velocity, start by trying to tweak this to get your chart to match live fire data

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.

truing-ballistics-data4

Field Firing Solutions, an extremely powerful program, has a feature to calculate ballistic coefficient based on observed drop and velocity data

Other Considerations

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.

truing-ballistics-data1

Other things to check are highlighted in green like sight height, the range unit (meters or yards?), you can also use a temperature velocity table to account for velocity differences due to temperature changes

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!

truing-ballistics-data2

The Applied Ballistics Kestrel and accompanying profile loader allow for tweaking of muzzle velocity, ballistic coefficient, and inputting temperature velocity tables for accurate data uploads to the Kestrel

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.

truing-ballistics-data3

The Applied Ballistics Phone app has a dope truing function, plug in known drop at distances and it will calculate muzzle velocity based upon those numbers

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!

Wrapping Up

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!

Owner and Proprietor of AccuracyTech, LLC. Rich is a Firearms Enthusiast, Precision Rifle Competitor, and Writer. He is committed to bringing readers quality reviews and articles related to the Precision Shooting Sports. If you have any questions for him, please use the contact form on the site.

Comments

  1. Rich, I’m likely missing forest for the trees here. Applied Ballistic android app giving me a fit. Finally got good dope on paper with 5 shot groups out to 530 yards. But at 900 group center is 7-8″ high. Thought I would tweak velocity so changed 2750 fps, corrected velocity per AB app after using truing feature. In JBM all lines up, till I change velocity of course, and I took it down to 2715 fps to get the 900 yard drop in inches down 7″, which would center group. But of course now the holdover given is 7.6 rather than the 7.3 I was using, that gave group center 7-8″ high. So how do I true software with velocity?

  2. Author

    Dan, it sounds like you went the wrong way in JBM. If the rounds are hitting higher than expected, you need to INCREASE the muzzle velocity in JBM, not lower it. If the rounds are hitting lower than expected, then you DECREASE the muzzle velocity.

    Remember, if rounds are hitting high at distance there are a couple possibilities: 1-Your muzzle velocity is higher than you think it is, that’s why most apps and software true dope by adjusting velocity until the drops line up. This is the most common error, using bad muzzle velocity. Where did you get the muzzle velocity you are using? 2-The ballistic coefficient of the bullet is higher or lower than expected. This is possible, but unlikely, what bullets are you using and what BC are you plugging in? 3-The environmental conditions are wrong in the software. Don’t pull data from weather stations, the barometric pressure is usually corrected for sea level, and not station pressure. You want to use station pressure if you have the time and means to have the software load all the environmental data. If not, you need to approximate the Density Altitude with a chart. There’s an example in this article: http://www.accuracy-tech.com/density-altitude/ at the top of the page. Approximate your DA, and make sure the AB app on the phone is using density altitude. 4-Your distance is incorrect. How do you know how far the target is? Did you range it and you’re confident in the result? Or are you on a square range where it may be farther or closer than you think it is?

    The AB app has a truing feature, you can plug in the distance and the actual drop in MILs you needed to hit center, and the software will then correct all the dope based on that by altering your muzzle velocity to match. Make sure everything else is correct first. Make sure the AB app is using density altitude, read that right off your Kestrel (Do you have one?), and it should be pretty close.

    1. Author

      You can do it at any range but the farther and closer to transonic speed around Mach 1.2 the better. A lot of ballistics software won’t accept a value if it’s closer than specified in the program. I’d try for 800+ if at all possible.

  3. question I went to 500 shot a group corrected it on the turret to dead center at 500 yards put the info in my strelok pro app it gave me my new speed took it to 1050 and I was like 8 inches low what am I doing wrong

    1. Author

      How much of a correction at 500 did you make? It could be a lot of things. True it at 1050 not at 500. I need a lot more information to have any clue.

      What caliber, barrel length, what scope and in what angular measurement system, what was the app calling for at 500 and 1050 and what did you wind up needing, what bullet at what muzzle velocity, what did you use to get the muzzle velocity? Environmentals as well, temperature and barometric pressure?

Have a question or comment? We want to hear it!