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

[bellavite1]

In a previous thread I was made aware of the puzzling fact that rimfire's trajectory will often not match the trajectory calculations of ballistic apps.

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

However, although the measurements are pretty consistent, 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.

A few of you replied that this seems to be a pretty common issue with rimfire ballistics 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 Shooter App even have 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 (Blucore 100yds) 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?

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?

[kidicarus13]

Calling all astrophysicists and Mensa members.99548

[Wolfshoon]

Typically I have to adjust BC and drag model numbers to get rimfire to work in the kestral as published BC cartridge makers of 22 usually dont work or are for the bullet type I?m not shooting. They generally give one BC for all of their lines, regardless of what it actually is.

I?ll alternate between G1 and G7 after getting close with BC to see which works better. This gets it pretty close. BC is the bigger change that gets the curve closer than G1/G7. I always let the kestral acquire its atmospheric data as it calculates it on the fly for the area you are shooting in. Air density is a big deal for subsonics. Changes to temp and baro pressure do affect it.

This experience comes from years of rimfire shooting both Steel Dogs (35-200 yards) and Long Range rimfire events (to 421 yards) at CRC

Edit add: I normally use Strelok Pro for my calcs, but that app is russian and on the sanctioned list so it cant be downloaded right now.

To give an example of how much atmospherics affect this: I had to add a full half MIL to every value for Steel Dogs this month compared last month. Air density was different due to warmer temp and recent rain making the air less dense.

[bellavite1]

That is why I double checked my data outdoors:

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

Obviously, if I am indoors at Blucore, 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.

[Great-Kazoo]

Calling all astrophysicists and Mensa members.99548

It's easy to see, the crux of the biscuit is the apostrophe

[20X11]

factored in Air Density?...goes beyond pressure...
https://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

[bellavite1]

Supposedly Shooter App adjusts for that based on GPS (Location settings).

I have been asking the same question on Sniper's Hide and the best response I got so far seems to be this:

"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."

So it seems that adjusting the BC may be better than adjusting the velocity to match reality.

The question is to what degree...

With my 300BLK supers we are talking about my point of impact being about 2" high at a 100 yds when the actual velocity is plugged in.

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...

[Tim K]

Here's another way to think about it. The ballistic engines are all pretty good these days and should yield good results given good inputs. Chronographs are so good these day I say we can accept the results as accurate. The atmospheric data is almost certainly close enough. What's the only input we can't measure directly? BC. Our only option is to use the published numbers. I rarely get any discrepancy in my predicted data and what I see on the range, but if I do have a major problem, BC is the first place I look.

I'm a huge fan of Applied Ballistics. Litz has measured many bullets with his radar setup. In the Kestrel with AB or the AB Quantum app, you can access those "BC's" he measured as AB Custom Curves. I've yet to find one that didn't work nearly perfectly. I've had discrepancies, but they've always turned out to be user error of some sort on my part. The AB custom curves eliminate the need to adjust BC as velocity changes because they are direct measurements of the bullet in flight, not a scaling factor applied to a generic curve.

Don't disregard the effect of smaller changes in density altitude on subsonic projectiles. For example, I've had my data at 220y change by a solid 0.5mrad over the span of an hour or two at a .22 match. Check data first thing in the morning at 220, kestrel predicts 8.0, make good hits. Shoot that stage later in the day and the kestrel says 8.5. Dial it up and make good hits.

Also, be wary of wind direction. Crosswind jump can really fool you. Test for yourself in your app. Calculate a solution for 10mph wind going left to right wind and compare it to the same but with the wind from right to left. On a .22 at 300, it's nearly 0.4mrad difference in elevation. So, 4". I can sometimes be a big deal.