Brent, I really don't know about the algorythims. The G1 funtion is the one the Infinity uses, BUT the program can use the different BCs at different velocities, most programs cannot. There is somewhere in the web a version of the JBM ballistics program altered to use different BC at different velocities, and not surprisingly the results are very similar.
Jeff, Sierra tests their bullets in a 300 yds indoor range, and they mimic the velocity decay by downloading. As you probably know this is not the best way to find out real BCs because the spin rate, yaw angle, precession cycles, etc. are not the same, but I understand the differences are minor. A few bullets like the 175 and 190 SMK were tested in military proving grounds, real life extended range tests with doppler radars, yadda, yadda, and this data is of course known to Sierra and matches very well their Infinity program. I've confirmed it myself up to 1000 yds with good results, if your program is giving 31" error then there is something wrong [img]images/icons/confused.gif[/img]
I'll post the link to the modified JMB program when I find it, so your can use it to compare with your own data.
Brad's program at JBM uses a very massaged G1 drag model. Brad is extremely knowledgeable about ballistics. He used to have an outstanding ballistic program called JBM On Target but quit selling it back around 96/97 time frame. If you ask him, he will tell you that he uses the same calculations that have come out of Aberdeen proving grounds. When using his on-line program using the G1 drag function, it has been designed/massaged to match the G5 drag model and actual data for the bullets tested by the US Army. Which a lot of times has been the various sierra match king bullets.
Like Brent said, a lot of these programs use a G1 drag model, but many have different mathematical calculations/algorithms. Iíve talked with some of the designers of these programs. Basically they worked with a particular bullet and load they were shooting and came up with a mathematical calculation that matched their actual data very closely. Then they designed and published their program around that data. So if you shoot a bullet that closely resembles the one they used to design their program, you will do very well. But remember they call it a G1 drag function.
Common sense still says that a boat tail bullet, a VLD bullet, flat base spire point and a flat nosed bullets will all have very different aerodynamic flight characteristics at ranges from 400 to 1500 yards. Using a single mathematical calculation designed around one drag model to calculate the exterior ballistic of all those different bullet types is a receipt for failure.
Those of us who are serious about long range shooting spent a lot of time shooting at long ranges. We also keep a very accurate logbook with all of our shooting data. Once you have that, it makes it easy to apply those numbers to a computer ballistic program and if necessary play with your data inputs so that you can closely match your actual data.
Distance is not an issue, but the wind will make it interesting!
Thank you so very much.
I would be real unhappy if I had a Ground hog in my sights and it got away due to using the wrong setting when caclulating my drop chart.
By the way I saw my first LIVE grong hog yesterday. It is to bad I was on my way to work a MONSTER TRUCK show at the Brice Jorden Center in State Collage Pa. Grave Digger RULES!!!!! [img]images/icons/grin.gif[/img]
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Brent, I really don't know about the algorythims. The G1 funtion is the one the Infinity uses, BUT the program can use the different BCs at different velocities, most programs cannot. <HR></BLOCKQUOTE>
The reason that the Infinity uses increasingly smaller BCs is to correct the G1 drag function output. This function will produce drag results that are much lower than actual, hence the need to drop the BC along the trajectory. The fact that Sierra has to patch the trajectory in this way should be demonstration enough that the G1 drag function used by itself is not a good fit for small caliber bullets.
But, as the guys have said, better drag functions use smaller BCs and that is a marketing problem.
For the most part the military doesn't really use BCs and Drag functions anymore (in talking with Bob McCoy before he died). They use CD versus Mach Number. Of course the ratio of the drag function and BC gives you CD (with some constants and a velocity term thrown in) so mathematically it doesn't really matter which G function you use as long as you have the right BCs (that's the real trick). Of course if you're using 10-25 BCs you might as well use CD and Mach number. As for not being accurate down range, it sounds like the low velocity BCs aren't correct.
My online programs use some old second order curve fits to published G functions. My MPM program doesn't use drag functions at all except in output. My personal code, PM, is a point mass program and it uses linear interpolation into the ARL (used to be BRL) tables of G functions as a function of mach number.
The real utility in drag functions and the reason that you want to find the "right" one that you can limit the number of BCs you have to use. As an extreme example, if you bother to create a drag function for the 300 gn MK, you could use it over the whole velocity range with a BC of 1.0. Of course your drag function is really just the CD as a function of mach number (with the same constants and the velocity term).
Berger sent me some data for their 210 grain VLD. It showed BCs as a function of mach number, velocity and drag function. The G7 drag function varied from 0.316 to 0.338 over the range of 1500-3500 f/s. The G1 BC varied from 0.590 to 0.715.
Gee, I wish I'd said that! [img]images/icons/grin.gif[/img] Thanks for the illumination Brad.
FWIW, I think that by the time the average shooter reads/hears the formula for BC the eyes start to glaze, and the marketing types say "Got another one!" In their shoes I'd cast my soul aside and kneel before G1, "Magnum", and "bigger is ALWAYS better." Snake oil by any other name is still snake oil.
Thanks JBM, that makes sense.
Do you perhaps remember the site where there is a duplicate of your trajectory program adapter to imput several BC at different velocity ranges?
Now, does anybody thinks you are going to find a better "fit" than the G1 fuction for the 190 SMK (only 3.4% BC variation)? Seems highly unlikely. If you use any other curve you'll have to "patch it" some more [img]images/icons/wink.gif[/img]