G'day, When 'massaging' input figures in your ballistic programs to match true drops or true come ups do you modify the velocity input or the BC input to get best trajectory fit? Thanks, Matt

I have found that if you must make changes then its better to alter the bc. The velocity, if measured with a chronograph, is a given measurement and needs to remain that way. Changing velocity gives a false indication of your bullets path. Whereas the bc is a calculated figure and maybe prone to error. If you dont have chronographed figures and you are just reading from a loading manual, then you could change either because its only guess work anyway. There are too many variables this way.

You want to work with real data for at least three different ranges, A "0" range of 100 to 300 yards, a far range that is as long as you can get and still shoot a good group ,and a mid range that is about in the middle. Both chronagraphed speed and manufactures BC can be off slightly so I play with both on the program. First, print a drop chart (Best of the West program) using your veolocity and the Mfg. BC. Make sure all the input data is correct, ie. elevation , temperature, scope height, click value. Be sure to enter your "0" range . Do not enter far range value at this time. Click calculate. This will give you a chart of where you " should" be. Now this is where things get fun. You can use this data to get you on paper to collect your real data. Your original "O" will remain the same. Your long and mid data are the critical parts. Write down your real 1000 yard data (or whatever distance you are using). Hopefully it is close to the calculated data but what is important is the group average that you have shot. If the chart called for 22 MOA, and you are at 23, (Convert to clicks)record what your gun is doing . Do the same for the mid range. Now go back to the program and enter your real long range data , distance and clicks into the program. By default the program, will float the BC when you click calculate. Now look at your midrange data compared to the new chart. High or low indicate your BC or velocity need to be different. Be sure to write down your initial data and conditions and go back and play with the program by putting the BC back to the original and float the velocity. By playing with the numbers, you can get the chart to match the mid point data. Now your arc of flight will match the drop chart and will be very precise. For big game, a couple of clicks off is OK, but for varmets ,try to get as close as possible. The closer you are also affects where you will hit at 1400 or1500 yards. If you have defined the arc exactly , it will be spot on all the way. Good luck! Post your data and a bunch of guys on here will help.

Thanks fella's, Your replies confirm what I thought. I have a chrony to lock that variable down. I have a 100m and 220m paper range and steel plates at 680m and 810m at the bench at home, and 910m and 1000m steel plates at a neighbours down the road. So far I haven't been quite able to get a perfect match by varying velocity in the Ballistic's App on my iPod, but I will now try modifying the BC to try to get a better fit. paphil, I think you right about getting best match on the further distances because at closer range one click out doesn't mean as much on deer sized game. Cheers, Matt

The advice given is fine. But, keep in mind... Some published BCs are very accurate and some chronys are not accurate due calibration or certain conditions. There are other conditions that have a bearing such as - unaccounted for wind foreward/backward/sideways and ...up/down even on flat ground - wind vs spin drift - coreolis - temp/pressure/altitude - parallax - shooter position/hold - precision of the scope adjustments throughout the full range of travel ...to name a few -- richard

I have 3 chrono's and all three read + or - 100 feet of each other. Im a fan of the X-ball program and always end up changing the BC value to make things work.

Huntinco, That's fine. And, it will work for some of us some of the time. However, altering an accurate BC to make the trajectory fit your curve for one complete set of conditions that you may or may not have measured precisely is only good for that particular set of conditions. If temperature or pressure or any other variable changes, then your altered BC may no longer get you what you need for a precise shooting solution at longer ranges. If you've been able to determine through good science that certain published BCs are incorrect, please let us know which ones. thanks, Richard