Ballistic Programs Not Matching Actual Dope?

[Tim K]

I need scope height above the bore, temp, barometric pressure, zero range and Humidity if you have it. Post up your known drops at the various ranges and I'll fiddle with it.

[Delfuego]

It seems the only effective way is to find a program that allows me to input a hand full of known distances and there corresponding drop, and let the computer do the rest. I don't even know if such a program exists,

Dude! I just gave you the answer you are looking for. Shooter does it through that applet. The program works fine. The input data has been off, so the out put data is off. Use Litz G7 BC's if you can and try it again.

Or try this:
http://www.jbmballistics.com/ballist...culators.shtml
Same curves different application and free online.

[asmo]

It seems the only effective way is to find a program that allows me to input a hand full of known distances and there corresponding drop, and let the computer do the rest. I don't even know if such a program exists,

Certain, higher end, ballistic calculators (Exbal, ATrag, etc) will let you put in your actual shot placement at various ranges and then work backwards to give you a corrected reading.

Your not making friends.

And if BC is different at varying speeds then who knows what it actually is.

It varies at different velocities - but its mathematically possible to predict your BC at various speeds. That's why programs like 'shooter' allow you to put in multiple BCs - and they smooth the transition over the flight time. High end calcs don't smooth but give you a precise BC at a precise measurement with a precise velocity.

[ray1970]

Although a chrono could be slightly off as far as actual velocity, just using published numbers for a given load is probably way off.

A ballistics program is like any other program.... Put garbage in, get garbage out.

Find a buddy with a chronograph and see what your velocities really are.

[C Ward]

760 is very temp sensitive and will give MV swings up to 100 fps or more in some case's .

Box test the scope and physically confirm the adjustments through the whole range of expected use .

The ballistic programs are still a estimate based on a model , way better now than the past but still an estimate . Calculated data should always be confirmed .

And like any program garbage in garbage out , the more specific and exact your inputs the better .

[bogie]

I do not own a chrono. And from what I have gathered they are very finicky and only provide the shooter with a warm and fuzzy, not accurate muzzle velocity. I agree Tim K that I need to find an effective working program. Writing down dope works great when your going to the same range with same elevation, temperature, humidity, etc, etc. However gathering dope the old fashion way does me know justice if say I want to shoot effectively as a competitor at multiple different ranges.

I suppose the Chrono will get me in the ballpark and I can tweak MV from there to get the right bullet drop out of my ballistic program, but I have literally inputted MV from 2300fps at 10fps intervals all the way up to 2800fps and nothing matches my actual dope(along with tweaking BC). It seems the only effective way is to find a program that allows me to input a hand full of known distances and there corresponding drop, and let the computer do the rest. I don't even know if such a program exists, I guess nothing beats just getting my ass out to the range and practicing. I am just trying to avoid wasting money and time doing something the wrong way. Just trying to figure out how guys are getting drop charts in their ballistic programs that are getting them within a few inches at 1000yds.

TIM K I am using Hornady Match Brass, 48.5gr WIN 760(its all I can find), 155gr BTHP Nosler Custom Competition with BC of .45.
And if BC is different at varying speeds then who knows what it actually is.

Maybe I am just over thinking.

Have you considered a heavier bullet? Your lighter bullets will transition from supersonic to subsonic flight, which will change the ballistic path accordingly. e.g. bullet drag changes due to supersonic shockwaves, etc. 168 or 175 gr will maintain supersonic velocities out to further distances.

[C Ward]

Bullet weight has nothing to do with the BC of the bullet . The BC is a function of length and shape , the weight is a byproduct of this because longer bullets are typically heavier .

The Sierra 155 and 175 Match Kings and the 155 Scenar all have effectively the same BC number of .495 .

BC is what matters because that is the model for how well the bullet fly's.

[bogie]

Bullet weight has nothing to do with the BC of the bullet . The BC is a function of length and shape , the weight is a byproduct of this because longer bullets are typically heavier .

The Sierra 155 and 175 Match Kings and the 155 Scenar all have effectively the same BC number of .495 .

BC is what matters because that is the model for how well the bullet fly's.

I didn't want to get technical but I'll clarify. Look at the equation for BC, which is directly proportional to Mass. It can be rewritten as a function of the density (p), length (l) and drag coefficient (Cd). The equation shows that mass increase is proportional to BC increase, or as you said when the bullet length increases.



Alternatively BC is written for bullets, and again, is directly proportional to mass:



To get to my point, when you transition from supersonic to subsonic flight the aerodynamic characteristics change. These characteristics are described by the drag coefficient (Cd). The equation for Cd is shown below. The density (p) of the fluid (air) is higher during supersonic flight due to compressive shockwaves around the bullet. Note also that the bullet drag changes with the square of the bullet's velocity relative to the fluid velocity.



There is inherent transonic instability around Mach 1 and a resulting transition in these parameters. The ballistic coefficient is constant in a simple model, and doesn't take into account the many nonlinear effects at the transition from supersonic to subsonic flight. Hence when a bullet transitions to subsonic flight the air density and relative velocity see a significant change. This influences BC, which influences the point of impact. Therefore, it is desirable to maintain supersonic flight through a bullet's entire trajectory, as is typically accomplished with a heavier bullet.

[C Ward]

Explain why the 115 DTAC and the 142 SMK both have a better BC than the 175 SMK then or why the 2 diffevent 155 SMK's have different BC's then .

They're way more examples I can list , weight does not equal better BC .BC is bullet shape and length .