How can I make my ballistic tables agree with real world results?

R

Ruben Nasser

New member
Hello!! I know many of you guys are good at ballistics, so I'm asking here for your help. I have an old ballistic program (Sierra 3) that gives me a wealth of data for trajectories, wind deflection, etc. Recently I started comparing the computer results with real data obtained from shooting some rifle loads at 300 plus meters, and there were some differences. Since I can get the muzzle volocity from my cronograhp, then the ballistic coefficients of the bullets must be off. How can I correct this and make more accurate predictions? How accurate are the manufacturers BC's? I read some interesting stuff about this in reloading manual, at the "how bullets fly" web page, and I'm always reading the very interesting stuff from Rick Jamison in Shooting Times, but the more I read the more humble I become!!
 
The only surefire way to know your trajectory is to shoot at all ranges in as many conditions as possible, record the data in your log book, and create your charts from that.

BC's from the manufacturers are close, but not exact. To determine the exact BC of your bullet you need an accurate chrono at the muzzle and one downrange. You calculate the BC from the velocity lost over a known distance.



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Cry "Havoc!" and let slip the dogs of war.
 
Don't forget to include altitude in your calculations. I live in Utah and my long range shooting area is at 5100 feet elevation. It makes a large difference over sea level book values. I use a Sierra Infinity ballistics program and it matches quite well with my real world results.

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Get your 1911s and AR15s while you still can!
 
Watch-Six brings up a very good point. Be sure your program takes altitude into account instead of using an average for the US, for example. If I recall correctly, Asuncion is practically at sea level and so your bullets will slow down faster than they would at an "average" elevation.
 
Thanks for your comments!! I take into account local temperature and altitude, and also the wind conditions. Since I don't have sophisticated chrono to put the skycreens downrange and get the corrected BC's at each distance (since apparently published BC's are based on the G1 model, that's why you get different BC's for each velocity range) I was wondering if there is some sort of mathematical formula based on observed tarjectory rather than real downrange velocity. Or maybe I should take Rex Feral's advice and then play with the computer program's BC's until I'm close to match the real world trajectory. Of course then I would be working just for the fun(?) of it!!
 
The BC also changes dramatically when the bullet goes subsonic, if you are shooting something that stays supersonic thru it's entire effective range you will not have to worry about this, otherwise you will have to take this into account.

I have to stress once again, the only way to really know what your load is going to do at extended ranges is to actually *SHOOT* at those distances. Ballistics programs can come close, but if you are serious about hitting your target, close is not good enough.

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Cry "Havoc!" and let slip the dogs of war.

[This message has been edited by Rex Feral (edited May 18, 2000).]
 
Even though it is possible to derive the BC from velocity lost down range, BC is a constant and does not change with velocity. It is difficult to calculate because a key component is the "form factor", which is an approximation, however close it may be. BC = the ratio of a bullet's weight to the product of the square of its diameter and its form factor. Stated another way, it is the ratio of a bullet's sectional density to its coefficient of form (form factor again).

Nothing in there about velocity. So, I think, Rex, you are correct in your first post,where you said the BC can be determined by measuring the velocity lost, but incorrect in the second one where you said the BC would CHANGE with velocity.



[This message has been edited by sensop (edited May 19, 2000).]
 
You're probably right sensop, I can't remember where I picked up the tidbit about a bullet having a different BC when it goes subsonic, thought it was a reliable source though.

I personally have never had any problems with my tables, 'cause I don't bother with ballistics programs or tables for real world data; I take the damn rifle out and SHOOT IT at 100, 200, 300, 400, 500 etc, etc, etc. Then I SHOOT IT some more when I have a steady wind and build all my charts that way! This has benifits other than accurate drop & drift tables as well, it gives you practice!

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Cry "Havoc!" and let slip the dogs of war.

[This message has been edited by Rex Feral (edited May 19, 2000).]
 
According to what I read, it is not that BC change with velocity, but rather the coefficient of form of the different bullets don't correlate very well to the "traditional" G1 model, and changing the BC according to different velocity ranges is an attempt to mimic the vehaviour of different bullets with very different form factors in a way that most people not very familiar with ballistics can grasp more easily, otherwise comparing those bullets would be an "apples to oranges" kind of comparison, deceiving for most people.
 
Ruben,

Got a source?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>... changing the BC according to different velocity ranges is an attempt to mimic the vehaviour of different bullets with very different form factors in a way that most people not very familiar with ballistics can grasp more easily, ...[/quote]For bullet going downrange, there is no rationale for "mimicking the behaviour of different bullets with very different form factors". Why would one need to plot the trajectory of different shaped bullets down the same path of flight, except for a comparison of the entire trajectory of different bullets, not a different ogive every so many feet, for the same bullet. 'Just doesn't make sense.

[This message has been edited by sensop (edited May 19, 2000).]
 
Sensop, maybe I did'nt express myself very well. What I tried to say was that conventional ballistics tables take one bullet with a given form coefficient (say, G7, very low drag bullet) and try to express its path using the ("traditional", originally Krupp's) G1 form factor. Since the paths won't match, a different BC (G1) is used at different velocities to get them pretty close at normal ranges. Apparently this is due to tradition (in earlier days before sophisticated computers it was easier to use existing and very detailed G1 tables and apply correction factors), and the fact that manufacturer's want a shooter to compare two different bullets with very different form factors (say, blunt nosed and SBT) in an easy way, comparing directly the BC's (which otherwise would be a bit complicated). For example a VLD bullet with a very efficient form factor may have a lower BC (by G7) than a spitzer (by G1), yet obviously is more aerodinamic. You can read about this far better than I can try to explain at: External Ballistics: http://207.181.16/johns/ballist.htm (FrFrogs page).
 
Thanks again. The first link doesn't want to hook up on my end, but the Fr. Frog site looks very good! I will read and cogitate, as they say.
 
One thing to keep in mind is that the BC of a bullet can only be accurately (exactly) determined in each individual rifle/bullet combo.

The smoothness of the barrel, rate of twist, bullet tip deformation, obturation of the bullet - all play a part.

Sensop - there is a lot of work on the sonic/sub-sonic turbulence problems. I'm in a hotel room, but when I get to the house I'll dig around for some articles in Precision Shooting for 'zacts.

Giz
 
There are several goods sites dedicated to ballistics, but one that is definitely worth a look is the article "how bullets fly", you can find it at: http://www.snipercoutry.com/ballistics
I find all information on ballistics fascinating, but sometimes it can be very "arid" reading. Probably if you are a shooter there is only so much you have to know, beyond that point it becomes a hobby in itself.

[This message has been edited by Ruben Nasser (edited May 20, 2000).]
 
Gizmo,

I'm an input monster. Send what ever you got my way when you get a chance.

Ruben,

Thanks for the link. Not dry to me.
 
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