Truing Ballistic Calculator, Velocity Errors?

[TheNatural21]

I'm trying to true my ballistic calculator with some actual DOPE. I only have good data to 500m, but I have that for a total of 4 loads across 2 rifles. Without truing I get some results from the calculators that are waaaay off - always more elevation, and sometimes as much as 50% more elevation than I actually need. I'd like to travel a bit and get some longer range shooting in (pretty dense bush near me), but it would be nice to not spend all day messing around with try dope that's 5 mils high!

I've tried several different calculators to check my results (they all give similar or the same solutions). Currently using the Hornady standard ballistic calculator on the website, but also tried the Hornady app, JBM ballistics, Strelok, etc.

My data is muzzle velocity from a Caldwell chronograph, environmental info from a kestral, measured scope height with a caliper, and measured targets at known distances. So far I'm just using the BC from the box (some loads G1 is all I have. If I have a G7 I use that).

I've also been using Gordons Reloading Tool for a while. I have been very impressed that with several loads including different cartridges, powders, bullets, etc, that it has always predicted within 50fps of my measured muzzle velocity, and usually about +/- 20fps. I have manually measured the case capacities in grains of H20 for an input (sometimes a bit different than the default value), and I measure my powder charges on an RCBS matchmaster. My "good" loads I reject any charge that doesn't fall within an ES of 0.4 grains, so usually single digit velocity SD for a 5 shot group. I thought I had everything pretty precisely controlled and my numbers were matching up good between the software and the chrono!

If I true the computer by adjusting muzzle velocity, I have to increase my measured velocities by 8-19% for the computer to match my DOPE. This is 4 different loads in 2 different rifles.

Is it really possible that my chronograph is reading 250-500fps low, even though it lines up with the numbers from Gordon's Reloading Tool (which I believe is similar to quickload, although I've never used quickload)? Is there something else I'm missing?

I ruled out scope tracking errors with a tall target test and trying this with 2 different rifles with different optics.

Loads tested are:
308 Win with a 125 TNT
308 Win with a 180 Accubond
308 Win with 185 Juggernaut
6.5 Creedmoor with 123 Scenar

 

[Ronws]

So, is it off, even if you input an average MV from the chrono, rather than box MV? Most people treat the box as the ideal but do not rely on it, cart blanche.

 

[TheNatural21]

So, is it off, even if you input an average MV from the chrono, rather than box MV? Most people treat the box as the ideal but do not rely on it, cart blanche.

They're all reloads, so no box velocity numbers available. I'm using the Chrono numbers for velocity.

The bullet box is where I'm getting the BC.

 

[LR1845]

This would be a better place to post your question and if you read through some of the sticky’s there you may find an answer to your question.

Ballistic Calculators & Field Results

Open discussion straight from the range on exterior ballistics.

www.snipershide.com

 

[Ronws]

They're all reloads, so no box velocity numbers available. I'm using the Chrono numbers for velocity.

The bullet box is where I'm getting the BC.

Okay. I know that in Strelok Pro, it can calculate a true BC based on your real collected data.

 

[waveslayer]

AB, Trasol allow you to true your MV. Biggest factors will be, garbage in garbage out. Make sure you input all the settings accurately and nothing is off like scope over bore height.

Also, is your scope tracking properly. Have you done a box test and a return to zero? That could be part of the problem

 

[TheNatural21]

AB, Trasol allow you to true your MV. Biggest factors will be, garbage in garbage out. Make sure you input all the settings accurately and nothing is off like scope over bore height.

Also, is your scope tracking properly. Have you done a box test and a return to zero? That could be part of the problem

Scope height measured with calipers, tall target test and 2 different scopes on different rifles to verify tracking is not the issue.

 

[TheNatural21]

After thinking about it all night I got it figured out! I was trusting the distance signs at the range but they are not labelled correctly. The signs say meters, but they're closer to those numbers in yards and still not quite right. What's labelled as 300m measures out to only 268m (293 yards) from the firing line in Google Earth! Other targets not as far off, but also not as posted (Including my 100m zero, which is actually at 102 yards). These differences at short range apparently make a huge difference. As waveslayer said, garbage in garbage out.

If I use the distances I measure in Google Earth with my dope for those targets, they line up with the ballistic calculator solutions pretty close. Some slight differences for a couple loads that will require truing, but the very worst computer solution is only off of my dope by 0.3mils now.

 

[chevy_man]

I true using the multi-bc available in strelok pro.

Hornady bullets work great with their app and the custom drag curves they've integrated. BC does change as the velocity changes.

I also bought AB and it has been spot on with the custom drag curves. I just can't get over how clunky the app is on Android.

I never use G1, G7 only. Even if it's not listed on the box Google will find it. For rimfire I started getting a lot better data using the RA4 profile.

Always verify distance with your own rangefinder. No sense gathering data unless it matches what you're going to measure off the range.