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.