Box velocity vs chrono vs trued to distance velocity

[WoolyBugger]

I picked up a Garmin Xero the other day and went out to test velocity and sd on my first batch of hand loads then shot box Hornady Match 6.5cm 140 gr eldm. Box is 2710 (i know this isn’t going to be accurate) and when I shot this ammo out to 1000 yards a few weeks ago before I got the Garmin, my calculated velocity was 2607, this gave me spot on hits from 100-1000 and anywhere in between. When I shot it over the chrono they were averaging 2688 over 5 shots. This is same lot number as the boxes I trued my ballistic calculator with. Pretty similar environmental conditions and maybe 30 more rounds on the barrel. Any ideas why my calculated velocity would be this far off?

Edit: I know 5 rounds is a low sample size but didn’t have much time to get this done on a lunch break

 

[freezerfull]

Out of the gate I’d guess the ballistic coefficient is likely lower than the box states. Generally I like a known velocity and adjust my dope by modifying the BC. BC of a projectile will also deminish as velocity drops. Usually out to 1k you can determinate a nice split the difference BC to keep things simple and not use a tiered BC calculation and still make acceptable hits.

 

[blkqi]

Since you have the Xero I would probably true the BC as stated above.

 

[WoolyBugger]

Out of the gate I’d guess the ballistic coefficient is likely lower than the box states. Generally I like a known velocity and adjust my dope by modifying the BC. BC of a projectile will also deminish as velocity drops. Usually out to 1k you can determinate a nice split the difference BC to keep things simple and not use a tiered BC calculation and still make acceptable hits.

Thank you I’ll play with bc. Excuse my ignorance, new to 500+ shooting and def new to having a chrono

 

[Atao]

How far are you planning to shoot? If you drop is spot on from 100-1000 why change anything?
I had a similar issue last year. I used a labradar to get velocity but i my ballistic calculator was off.
I verified zero @100yds, then i adjusted my velocity to match known drops at 400, 800 and 1000. I ended up adding 35fps and now my ballistic calculator is dead on out to 1400yds.

 

[HuntHarder]

It's weird how we rarely see bullet BC's under stated. Almost always overstated

 

[WoolyBugger]

How far are you planning to shoot? If you drop is spot on from 100-1000 why change anything?
I had a similar issue last year. I used a labradar to get velocity but i my ballistic calculator was off.
I verified zero @100yds, then i adjusted my velocity to match known drops at 400, 800 and 1000. I ended up adding 35fps and now my ballistic calculator is dead on out to 1400yds.

I’m shooting nrl in a month targets out to 800. I’m not planning on changing my calculator for the boxed ammo since it’s dialed, but more so interested in why it’s not lining up so I can hopefully spend less time getting my hand loads trued up

 

[hereinaz]

Thank you I’ll play with bc. Excuse my ignorance, new to 500+ shooting and def new to having a chrono

Don’t play with BC unless you have to.

BC is dependent on velocity, so there technically isn’t “one BC”. If you look deep, bullet companies disclose the distance and velocity for the BC they give.

Hornady uses Doppler radar confirmed models—not BC for its 4 DOF.

Applied Ballistics uses Doppler to create a custom drag model for bullets in their library. And their mobile lab can create a personal drag models based on your rifle.

The ballistic calculations are based on MODELS not real life. They model the flight, its best to call them a prediction.

It doesn’t surprise me that one lower velocity can get hits to 1000. That means the bullet, for whatever reason, fired from your rifle slows down faster than predicted.

Just be happy you have a solution that works. Go shoot it and move on. It’s not a problem, it’s the way it is. It is very likely that no amount of tweaking BC and velocity will improve what you have now.

Believe the bullet.

The Garmin velocity is accurate.

 

[WoolyBugger]

Don’t play with BC unless you have to.

BC is dependent on velocity, so there technically isn’t “one BC”. If you look deep, bullet companies disclose the distance and velocity for the BC they give.

Hornady uses Doppler radar confirmed models—not BC for its 4 DOF.

Applied Ballistics uses Doppler to create a custom drag model for bullets in their library. And their mobile lab can create a personal drag models based on your rifle.

The ballistic calculations are based on MODELS not real life. They model the flight, its best to call them a prediction.

It doesn’t surprise me that one lower velocity can get hits to 1000. That means the bullet, for whatever reason, fired from your rifle slows down faster than predicted.

Just be happy you have a solution that works. Go shoot it and move on. It’s not a problem, it’s the way it is. It is very likely that no amount of tweaking BC and velocity will improve what you have now.

Believe the bullet.

The Garmin velocity is accurate.

Thank you for the information, always appreciate your feedback on here. No plans to mess with my calculator for the boxed ammo, will be shooting that at NRL in a month

 

[hereinaz]

Thank you for the information, always appreciate your feedback on here. No plans to mess with my calculator for the boxed ammo, will be shooting that at NRL in a month

Sometimes the environmentals can tweak your dope just enough to push it off if there is a big elevation difference between where you doped and where you shoot. So, believe the bullet if you are consistently high or consistently low, and don't get frustrated. Adapt.

It probably won't happen, but if there is a pattern over one or two stages, don't fight it. Just add a bit in or take a bit off. Also, once you get into alternative positions, if you haven't practiced with them, the different recoil impulse can sometimes make shooters miss high.

A way that I approach a stage, is that as the position becomes more "unsteady" with less rear support or the front support moves toward the magazine, I start aiming lower on the target, because I know that if recoil exploits me, I will miss high. When we are on a bench or prone, the rifle muzzle doesn't move up as much during recoil, so there is sometimes a shock to shooters when misses seem high and frustration can build.

Again, go with it if it happens to you. The degree to which the environmentals will mess with you is less than the degree to which the alternative positions and the pressure of the clock start messing with you.

Because of the dynamic that unsteady positions generally cause errors high (because that is the direction the muzzle most often travels under recoil because of rifle design--bore over butt stock), low misses are more often a failure to dial dope, general instability/big wobble, or a slight change in dope.

Again, in my experience, it takes quite a drastic change in DA to really affect DOPE.

Hope you have a great time at the match. Hope you don't mind the unsolicited rant.

 

[ddowning]

I'll second the above. The more correct your inputs are, the better your ballistic calculator will travel. For a set of conditions there are a lot of combos you can enter into a ballistic calculator that will line up with a given trajectory. If you only change the environmentals and the other inputs are off, you will not have the correct dope in the new environmentals. It does take a pretty big swing to cause a problem on nrl hunter sized targets.

Now that you have a Garmin you can get good velocity info. Adjusting the bc is sort of a crapshoot, but you can fine tune over time and then get extremely close. The other thing that effects things is that when temp changes, so does velocity due to powder burn speed changing with temp. Some powders are worse than others. Now you have a moving velocity, moving temp and because bc changes with velocity, you will have a tiny change in bc. This is why people are transitioning to custom drag models.

I made it seem sort of complicated, but it really isn't for nrl hunter. Guys with a lot of experience can probably take a winter and summer dope card and hit the targets out to 800 yards. It only gets more complicated if you make the targets smaller, the distance further, or travel to areas that have vastly different elevation and environmentals. Hopefully you don't mind the information. I put it out there to both reiterate how simple things can be at 600-800 yards and the things that start to get frustrating as you push past 1000 yards or want to shoot smaller targets.

 

[freezerfull]

I’ll add to my initial response and give my means and methods for developing a long term model for a rifle/bullet combo out to 1k. Hope this is helpful. For ELR, having a tiered BC based off velocity drop (drag profile) is much more important but for 1k and in I prefer a simple BC truing. The first part is only applicable if you hand load or have a large quantity of ammo from the same lot, for small batches of ammo or if your planned shooting is in similar temps then just chronograph a few shots in anticipated temperatures.

1. Chronograph loads at multiple temperatures. Make sure your ammo and rifle are at the ambient temperature. Some cheat this by using freezers or a warm truck cab to simulate environmental temp. I usually just shoot in below freezing temps in winter, hottest week of the summer and a few in between to gather data. This will give a decent velocity model for given temperatures with your load.

2. Input BC from bullet manufacturer into calculator. Also measure your bullet and input bullet length and diameter and rifle twist. For some reason Strelok had a few erroneous outcomes with length/BC inputs when there were length discrepancies. Then very accurately input all environmental conditions into calculator, extremely important is the barometric pressure.

3. With the correct environmental and muzzle velocity input into the calculator. Shoot your rifle at multiple ranges, with a know zero distance of course. Say 500, 800, and 1000. Now record your adjustments for center hits at each distance. What I do then is true my BC for each distance/required adjustment. Usually for most higher MV cartridges there is very little difference in the trued BC for these ranges. With slower cartridges I generally see a bigger drop of BC as ranges get further which is no surprise. This is where a drag profile or velocity tiered BC begins to matter.

4. If I want to use a simple BC not a drag profile I will do the following. Now with the trued BC best providing good hits at the intended spread of ranges I use that BC in my calculator.

5. Always record data each time you shoot: velocity and environmentals. Over the years I’ve had good success with using the trued BC for mid range shooting (out to 1k) working as long as current environmentals are input correctly.

For example:

Known velocity at ambient temp: 2850fps
Known pressure: 28.20
Calculated G7 BC: .307 Box BC was .316
This will provide MOA impacts from 100-1200 yards.

Often times I use the trued BC from the further end of the range spectrum, like 800 yards. This is better than the reverse as it will put you off say 2” high 400 yards, totally acceptable. If you used a trued BC from say 400 or 500 yards, you may be 12” low at 800 yards, totally unacceptable especially if your shooting at an animal.

 

[WoolyBugger]

I’ll add to my initial response and give my means and methods for developing a long term model for a rifle/bullet combo out to 1k. Hope this is helpful. For ELR, having a tiered BC based off velocity drop (drag profile) is much more important but for 1k and in I prefer a simple BC truing. The first part is only applicable if you hand load or have a large quantity of ammo from the same lot, for small batches of ammo or if your planned shooting is in similar temps then just chronograph a few shots in anticipated temperatures.

1. Chronograph loads at multiple temperatures. Make sure your ammo and rifle are at the ambient temperature. Some cheat this by using freezers or a warm truck cab to simulate environmental temp. I usually just shoot in below freezing temps in winter, hottest week of the summer and a few in between to gather data. This will give a decent velocity model for given temperatures with your load.

2. Input BC from bullet manufacturer into calculator. Also measure your bullet and input bullet length and diameter and rifle twist. For some reason Strelok had a few erroneous outcomes with length/BC inputs when there were length discrepancies. Then very accurately input all environmental conditions into calculator, extremely important is the barometric pressure.

3. With the correct environmental and muzzle velocity input into the calculator. Shoot your rifle at multiple ranges, with a know zero distance of course. Say 500, 800, and 1000. Now record your adjustments for center hits at each distance. What I do then is true my BC for each distance/required adjustment. Usually for most higher MV cartridges there is very little difference in the trued BC for these ranges. With slower cartridges I generally see a bigger drop of BC as ranges get further which is no surprise. This is where a drag profile or velocity tiered BC begins to matter.

4. If I want to use a simple BC not a drag profile I will do the following. Now with the trued BC best providing good hits at the intended spread of ranges I use that BC in my calculator.

5. Always record data each time you shoot: velocity and environmentals. Over the years I’ve had good success with using the trued BC for mid range shooting (out to 1k) working as long as current environmentals are input correctly.

For example:

Known velocity at ambient temp: 2850fps
Known pressure: 28.20
Calculated G7 BC: .307 Box BC was .316
This will provide MOA impacts from 100-1200 yards.

Often times I use the trued BC from the further end of the range spectrum, like 800 yards. This is better than the reverse as it will put you off say 2” high 400 yards, totally acceptable. If you used a trued BC from say 400 or 500 yards, you may be 12” low at 800 yards, totally unacceptable especially if your shooting at an animal.

So thats what all the buttons on the AB App do, I appreciate the information. Makes a lot of sense to do the custom drag in head. So when you at chronoing at multiple temp points, are you using the mv-temp table? Or what temp are you using to input your velocity used for the calculator?

 

[freezerfull]

So thats what all the buttons on the AB App do, I appreciate the information. Makes a lot of sense to do the custom drag in head. So when you at chronoing at multiple temp points, are you using the mv-temp table? Or what temp are you using to input your velocity used for the calculator?

Most ballistic calculator programs allow you to input multiple velocities with a referenced temperature. I know atleast AB and Strelok do. This way you can store your velocity/temp data and when you put the current temp in the program it will automatically estimate muzzle velocity at that temp as long as enough velocity/temp data has been stored in the program.

 

[wind gypsy]

Did you use AB for the calculator? With custom curve?

 

[WoolyBugger]

Did you use AB for the calculator? With custom curve?

the original calculator I used to get the velocity based on hits at 1000 was AB lite in the sig app for my binos.I don’t think it has custom curves. I have the AB quantum app now and working on getting kestrel so I can use AB elite in the binos

 

[Formidilosus]

There’s so much nonsense posted in here.


@WoolyBugger

How much change is in there at 800 yards say, with a 10 point change in the G1 BC (say .540 to .530)? Because that’s about the maximum difference in G1 BC’s rifle to rifle. All the ways- real G1, G7, CEM, 4dof- all of them produce functionally identical data in the sub 1,100’is yard ranges.
There’s a hint here- BC isn’t the reason your data is different.


How many shots did you confirm zero with? Have a picture of the zero target?


Have you live fired your scope tracking/adjusting and checking at least every 1 mil for at least 10 mil’s? Scopes easily and frequently do not adjust exactly .1 mil or .25 MOA per click through their whole range. It’s not unusual to see 2-5% error at 6-7 mils.



The most likely- as in a near certainty; is that your zero isn’t perfect, your scope may not be adjusting at 100%, and/or you didn’t account for environmentals correctly. RF binos have to acclimate to ambient temp- pulled straight out of a heated or air conditioned vehicle, or letting the sun, or kept in a bino harness on a cold day- they will read off.

 

[WoolyBugger]

There’s so much nonsense posted in here.


@WoolyBugger

How much change is in there at 800 yards say, with a 10 point change in the G1 BC (say .540 to .530)? Because that’s about the maximum difference in G1 BC’s rifle to rifle. All the ways- real G1, G7, CEM, 4dof- all of them produce functionally identical data in the sub 1,100’is yard ranges.
There’s a hint here- BC isn’t the reason your data is different.


How many shots did you confirm zero with? Have a picture of the zero target?


Have you live fired your scope tracking/adjusting and checking at least every 1 mil for at least 10 mil’s? Scopes easily and frequently do not adjust exactly .1 mil or .25 MOA per click through their whole range. It’s not unusual to see 2-5% error at 6-7 mils.



The most likely- as in a near certainty; is that your zero isn’t perfect, your scope may not be adjusting at 100%, and/or you didn’t account for environmentals correctly. RF binos have to acclimate to ambient temp- pulled straight out of a heated or air conditioned vehicle, or letting the sun, or kept in a bino harness on a cold day- they will read off.

Zero was done off a 15 shot group, reconfirmed the other day with a 5 shot group before moving on to test other loads. I do have a picture of that but not the original zero. I am pretty confident in my zero.
I have not done a tracking test, but say it was tracking off by 5%, Is there an input in AB, or whatever calculator being used, to take that into account? Or is it better to have a velocity different than your chrono that gives you an accurate solution? Environmentals are also a decent possibility, I’m not sure how accurate the sigs are.

 

[freezerfull]

@Formidilosus

I have a bit of a run on question. If your scope is verified to track, your zero is confirmed, environmentals are input correctly, height over bore is correct, velocity is known and BC used in your calculator is off a box of bullets yet your actual drops don’t match the calculations what would you recommend to be the metric that needs to be changed to allow your calculator to provide an accurate flight profile that can be modified to changing environments and temp adjusted velocities?

As far as I’m aware, it’s only the BC/drag profile that can be corrected that will provide good data as environmentals (including temperature adjusted velocity) change. If there is another metric that fine tunes my dope more accurately I’d like to know.

 

[BBob]

Is there an input in AB, or whatever calculator being used, to take that into account?

Yes

AB Quantum. When editing gun profile you'll find this under advanced settings. Most if not all apps have some way to adjust for it as well. In the video he touches on the elevation and windage correction at the 1:30 mark. I don't see a specific video for the adjustment though.

At about the 21 minute mark he touches on it again. I don't see detailed info in the manual either.

Here you go: Tall target correction factor.

https://appliedballisticsllc.com/wp-content/uploads/2021/06/Tall-Target.pdf