2+2=7, but WHY? Expectations vs reality.

[bellavite1]

I have posted this on a local Forum too, hopefully someone can chime in...

First and foremost, I am far from being an expert, and my gear is just entry level, so please bear with me if I am missing something obvious.

I have recently purchased an Athlon chronograph to measure more reliably the velocities on my rifles.

However, with my 22LR and 300 BLK bolt rifles, once I plug the velocity in my Shooter App, reality does not match the provided ballistic solution.

In order to reconcile the two I must enter a velocity higher than what my chrono says.
Once I do that, everything falls into place.

My understanding is that this seems to be a pretty common issue with rimfire ballistics Apps and just a few Apps will be able to calculate the real trajectory of a given rimfire load.
The consensus seems to be to tweak things until expectations match reality.

The Shooters App even has a "Velocity Calibration" option to do just that.

I have now confirmed that the same seems to happen with my 300 BLK bolt rifle.
Both subsonic and supersonic loads are plagued by this issue, with the supersonic loads requiring a tweaked velocity close to 300fps higher than the chronographed value.

Again, once the "correct" velocity was entered, I was able to consistently ring a 12" gong all the way to 330yds (probably could have gone further than that but I did not have the chance today).

I get the same results both indoors up to 100 yds and outdoors,so air pressure and temperature discrepancies are not a factor.

I am pretty confident in the chronograph readings since my 308 and 338LM are spot on.

This only seems to happen with 22lr and 300BLK, both calibers in sub and supersonic loads.

Sure, I have figured out the "right" velocity to plug into my app and I can now make my hits, but what is causing this?
Is it just a "glitch" with the App or is there some factor I am not considering?
The parameters I enter on any given load are:

Bullet Diameter
Bullet weight
Muzzle velocity
Atmosphere:ASM
Drag Model:G1
BC
Zero range

Am I missing something?

Shooter App acquires atmospheric data based on your GPS from the nearest weather station.

Obviously, if I am indoors, air pressure and temperature will be different from the data provided by the app (the app will give data for outdoors, at that location).

And yet, the indoors tweaked velocity worked quite well outdoors this morning with minor adjustments on the 300BLK Sub (1220 fps vs 1240 fps indoors).

The Supersonic loads required no further adjustment to the velocity.

I mean, it works.
But I wish I could understand the "mechanics" of it.

I used to think I lived in a Universe where 2+2=4...I guess that's not entirely true.

Can anyone PLEASE explain me how that is even possible?

 

[roostercogburn98]

Height over bore input?

 

[6.5SH]

Unless you know the stated BC was recorded using the ASM standard that is one error right there. ICAO is much more common.

Ballistic Program Inaccuracies

 

[Ledzep]

If you input what you know to be true MV (Garmin, Labradar, etc.), and you input the correct sight over bore and zero information, then the biggest reason you don't hit where the calculator tells you is because it has error(s) in the drag portion of the solution.

BC is an approximation of the drag. It takes a standard projectile (artillery projectile) and mass scales, and drag (shape) scales the Cd vs. Mach curve of the standard projectile and uses that new scaled Cd vs. Mach curve for the calculations to approximate trajectory for your projectile.

Because the Cd vs. Mach curve of any given projectile is EXTREMELY unlikely to be a perfect match along the entire Mach regime to that of a scaled standard projectile, there are errors. The way these calculators work, the errors calculated early on in the trajectory are carried through all of the successive calculations as it either time or distance step iterates to your requested distance. If the approximated Cd is wrong, then it will calculate the wrong retained velocity, which it will use with the next iterative step that has now a wrong velocity + another wrong Cd value and the errors build and build.

So when you grab a single BC number to input to the calculator, you are generating a scaled G1 standard Cd vs Mach curve that probably crosses over the true Cd vs. Mach curve of your bullet only at 1 or 2 places. Everywhere else along the Mach regime there is error. Most BC's are calculated by mfgs at Mach 2 to Mach 3 (typically, the faster, the higher the BC value, the better it looks on paper-- there is no real industry standard that EVERYONE follows). So when you input that BC value it is likely to be accurate (it will have errors, but they will be small) at high velocity because that's where it was calculated from. However, when you start using it down at around Mach 1-1.5 (.300 blk and .22 LR), odds are the actual drag of your bullet and the Mach 2 scaled G1 projectile BC of your bullet are not the same.

Side show: .22lr is very susceptible to changing shape (drag) in various different barrels, and the BC/drag out of them is wildly variable compared to centerfire jacketed bullets.

 

[alamo5000]

I'm not an expert but BC changes with velocity.

So...if you shoot a bullet with a advertisement saying X, that is calculated at a specific velocity.

Then if you take that same bullet and drop the velocity by 900 fps, the BC is no longer X.

And on top of that, the BC changes in flight. It might be X at the muzzle, but Y at 300 yards. This is more than likely compounded even more by the much slower cartridges in question.

The calculator being used may or may not take that algorithmic effect into account, but rather just keep it standard.

 

[bellavite1]

Thank you all for your replies.

Yes, sight height is accounted for.

Good to know about the ASM/ICAO, I was not aware of it.
I will change my input to that on my ammo profiles.

The CD portion also makes sense (I had to read it like 5 or 6 times over, but now I think I got it...):
Unfortunately Hornady only provides 1 BC for their bullets.

Not factoring all of the above would compound errors and would explain at least some of what I am seeing...

 

[Ledzep]

If you're using Hornady bullets, and they're in the 4DoF library, it's a no-brainer to use 4DoF. We've shot the bullets on doppler radar to get the exact Cd vs. Mach plot for that specific bullet (No BC involved, no G1/G7 standard conversion. It treats your bullet as its own "standard"). We also have the added benefit of a mass model of the bullet that allows us to much more accurately predict dynamic responses. Aerodynamic jump, wind, gyro stability, etc.. all are extremely accurately calculated.

However, if it's a bullet that's not in the library, or a competitor bullet that's not in the library, I would suggest adjusting BC based on how you're hitting (hit higher than calculated, increase the BC to match, for example). We haven't added many .22lr options to 4DoF due to the above mentioned bore-to-bore variation we've seen.

 

[alamo5000]

You should not have to plug in more than one BC. The calculator is either complex enough to adjust or it isn't. The newer more complex ones do the adjustments automatically.

Again, there could be a lot more going on that is above my pay grade, but this is just one possible thing.

Also if you look at Sierras listings for each bullet, they have BCs listed per velocity range. I only mention that to show you examples.

 

[bellavite1]

If you're using Hornady bullets, and they're in the 4DoF library, it's a no-brainer to use 4DoF. We've shot the bullets on doppler radar to get the exact Cd vs. Mach plot for that specific bullet (No BC involved, no G1/G7 standard conversion. It treats your bullet as its own "standard"). We also have the added benefit of a mass model of the bullet that allows us to much more accurately predict dynamic responses. Aerodynamic jump, wind, gyro stability, etc.. all are extremely accurately calculated.

However, if it's a bullet that's not in the library, or a competitor bullet that's not in the library, I would suggest adjusting BC based on how you're hitting (hit higher than calculated, increase the BC to match, for example). We haven't added many .22lr options to 4DoF due to the above mentioned bore-to-bore variation we've seen.

I'd rather stick to the same App, but I appreciate your input.

In this case, if I get this correctly, I should adjust the BC to match reality rather than enter a fictional velocity.
It would definitely make more sense, since the BC actually changes at lower velocities.

The question is to what degree...
We are talking about my point of impact being about 2" high at a 100 yds when the actual velocity is plugged in.
I have tried, for the sake of experiment, to adjust the BC from 0.398 to 0.498, with the actual velocity, and the elevation solution did not change.
Or was I supposed to go the other way (i.e. 0.398 to 0.298-still no change)?

I'm assuming I would I adjust it only at one specific distance, like 100 yds even though the rifle is zeroed at 25 yds?

 

[alamo5000]

I am just asking for clarification, but have you put your elevation in your calculator? Using sea level calculations up in the mountains can definitely throw everything off.

 

[Ledzep]

I'd rather stick to the same App, but I appreciate your input.

In this case, if I get this correctly, I should adjust the BC to match reality rather than enter a fictional velocity.
It would definitely make more sense, since the BC actually changes at lower velocities.

The question is to what degree...
We are talking about my point of impact being about 2" high at a 100 yds when the actual velocity is plugged in.
I have tried, for the sake of experiment, to adjust the BC from 0.398 to 0.498 and the elevation solution did not change.
Or was I supposed to go the other way (i.e. 0.398 to 0.358)?

I'm assuming I would I adjust it only at one specific distance, like 100 yds even though the rifle is zeroed at 25 yds?

Being that far off that close, even with subsonics... to me, screams of something else not being quite right. I'd be looking at your zero.

If you tell the calculator you're zeroed at 50 yards, it better be 50.0 yards and it better be spot-on at zero. The closer you do these things, the more precise everything needs to be. Saying, "Ahh .15" off, close enough" is twice as bad at 50 than it is at 100 than it is at 200, etc...

If you tell the app that the bullet crosses line of sight (zero) at a given distance, it runs the calculation so that it crosses at that distance exactly. Any error in your zero is a direct linear error carried through all the calculations.

 

[Baron23]

If you're using Hornady bullets, and they're in the 4DoF library, it's a no-brainer to use 4DoF. We've shot the bullets on doppler radar to get the exact Cd vs. Mach plot for that specific bullet (No BC involved, no G1/G7 standard conversion. It treats your bullet as its own "standard"). We also have the added benefit of a mass model of the bullet that allows us to much more accurately predict dynamic responses. Aerodynamic jump, wind, gyro stability, etc.. all are extremely accurately calculated.

However, if it's a bullet that's not in the library, or a competitor bullet that's not in the library, I would suggest adjusting BC based on how you're hitting (hit higher than calculated, increase the BC to match, for example). We haven't added many .22lr options to 4DoF due to the above mentioned bore-to-bore variation we've seen.

I do bc shoot bullets in the 4DOF library and find I often need to “true” the calculator .1 or .2 mils at 600 yds to get dead on the waterline.

Not a big deal to me and seems to be a common situation for shooters.

 

[Ledzep]

I do bc shoot bullets in the 4DOF library and find I often need to “true” the calculator .1 or .2 mils at 600 yds to get dead on the waterline.

Not a big deal to me and seems to be a common situation for shooters.

Yep that's what axial form factor is for. Every barrel (even muzzle devices in some situations) produces a slightly different Cd vs. Mach plot and we publish the average of the barrels we shoot the bullets through, so you may need to bump drag up or down a little with Axial Form Factor. The problem here is parsing out the components of error for the end user so that you can believe in what you're doing. Drag, MV, zero, scope over bore, environmentals, etc.. can all affect vertical POI. So, in order to say "It's definitely a drag error that is causing my vertical POI error.", you have to make sure the other stuff is locked down solid and are not contributing to the error. That's why we only let you adjust 5% either way because if you need to go more than that, something else is almost certainly wrong.

We get to cheat a little bit, and shoot our rifles over doppler to verify down-range velocity and match it with AFF in the app. Once that has been done, it's a bomb-proof profile and outside of wild wind conditions or screws loose between the eyes it's within .1 pretty much everywhere.

 

[bellavite1]

Being that far off that close, even with subsonics... to me, screams of something else not being quite right. I'd be looking at your zero.

If you tell the calculator you're zeroed at 50 yards, it better be 50.0 yards and it better be spot-on at zero. The closer you do these things, the more precise everything needs to be. Saying, "Ahh .15" off, close enough" is twice as bad at 50 than it is at 100 than it is at 200, etc...

If you tell the app that the bullet crosses line of sight (zero) at a given distance, it runs the calculation so that it crosses at that distance exactly. Any error in your zero is a direct linear error carried through all the calculations.

My zero is set indoors, with the distance actually ranged and the target placed at the exact spot (not 24yds, not 24.5, actual 25yds).
So, yeah I try to eliminate as many variables as I can.

 

[bellavite1]

I am just asking for clarification, but have you put your elevation in your calculator? Using sea level calculations up in the mountains can definitely throw everything off.

Yes, the App has location through GPS.

 

[Ledzep]

Take this or leave it. I noticed POI changes from our 100/200yd tunnel to outside in sunlight. I'm not an optical engineer and I don't have the know-how or equipment necessary to properly understand and test this to say anything with authority... I might even be wrong in correlating it to indoor vs. outdoor, but to the best of my knowledge that was my experience. YMMV.

 

[bellavite1]

As far as adjusting the BC to match the actual trajectory, should I enter a lower number to match the low velocity (i.e. 0.398 to 0.298) as the Sierra website seems to suggest?
How much lower should it be to show an actual difference?
The above change did not cause any change through my app.

 

[bellavite1]

Take this or leave it. I noticed POI changes from our 100/200yd tunnel to outside in sunlight. I'm not an optical engineer and I don't have the know-how or equipment necessary to properly understand and test this to say anything with authority... I might even be wrong in correlating it to indoor vs. outdoor, but to the best of my knowledge that was my experience. YMMV.

I thought of that but my tweaked velocity settings seemed to work outdoors as well.

 

[alamo5000]

As far as adjusting the BC to match the actual trajectory, should I enter a lower number to match the low velocity (i.e. 0.398 to 0.298) as the Sierra website seems to suggest?
How much lower should it be to show an actual difference?
The above change did not cause any change through my app.

Slower is lower.

 

[cas6969]

Is the 330 yard gong really 330 yards?

(the last time the math made me nuts (it wasn't by much, but it was the WHY???) , the 200 yard targets were 213 yards.

 

[6.5SH]

Thank you all for your replies.

Yes, sight height is accounted for.

Good to know about the ASM/ICAO, I was not aware of it.
I will change my input to that on my ammo profiles.

The CD portion also makes sense (I had to read it like 5 or 6 times over, but now I think I got it...):
Unfortunately Hornady only provides 1 BC for their bullets.

Not factoring all of the above would compound errors and would explain at least some of what I am seeing...

Ballistic Coefficient - Hornady Manufacturing, Inc

A-Tip®, ELD® Match, and ELD-X® Ballistic Coefficient Values Hornady originally published “800 yard”* average Ballistic Coefficient (BC) values for ELD-X and ELD Match bullets. This was done to provide the most usable BC from a trajectory prediction st...

www.hornady.com

G1 would be 0.669 or lower based on your velocity

 

[bellavite1]

Is the 330 yard gong really 330 yards?

(the last time the math made me nuts (it wasn't by much, but it was the WHY???) , the 200 yard targets were 213 yards.

228 to be exact, and I entered 228 in the App for distance.

 

[bellavite1]

Ballistic Coefficient - Hornady Manufacturing, Inc

A-Tip®, ELD® Match, and ELD-X® Ballistic Coefficient Values Hornady originally published “800 yard”* average Ballistic Coefficient (BC) values for ELD-X and ELD Match bullets. This was done to provide the most usable BC from a trajectory prediction st...

www.hornady.com

G1 would be 0.669 or lower based on your velocity

So, is there an actual formula that will allow me to calculate the proper BC?
Actual velocity on the 208 ELD is 1098 and for the 150 FMJ-BT is 2004.