Just reading a post of 308 WIN's on another thread here, I figured I'd do a little typing and "maybe" help fill in some blanks that some may have concerning BC's, drag models and the like. [img]images/icons/grin.gif[/img]
Anyone, feel free to add to, question, or downright correct me here. [img]images/icons/wink.gif[/img]
In a nutshell, the different drag curves will take a trajectory curve and either flatten the arc, or the opposite.
The G1 (SAAMI Standard Model) drag curve produces the least flat arc of the three commonly used models; the G7 is the flattest, with the G5 in between.
What to look for:
If you have a very accurate average MV, and 100 yard zero, then you get your drops out to 1000 yards or so, each 100 yard drop should perfectly match with a ballistic program from 100-1000yds using the G1 model. You will need to adjust the BC to fit the 1000 yard drop to match yours first, using your KNOWN MV and atmospheric conditions (VERY IMPORTANT).
If it does not match, look to see if the arc is actually flatter in the middle somewhere than the "predicted" drop indicates, then choose the G5 and modify the BC to fit at 1000 yards again. Some programs will convert the BC for you, it must still match at 1000 yards though. Adjust as necessary because you know the other variables. If this is still not flat enough in the middle somewhere, move to the G7 model and repeat. One should very closely model what you are seeing in the REAL world.
Each of the G1, G5, and G7 curves model a specific bullet shape, and some programs actually modify the G1 curve and come close to mimicking the G5 to G7 bullet form, some are far worse than other and do NOT tho.
One curve will never fit all bullet shapes and this is fact. For the utmost accuracy in a program modeling your "actual" trajectory curve, selecting the appropriate curve is recommended.
Atmospheric conditions and MV changes due to temp or fouling changes -
The drop data is only valid in the conditions you tested it in, but once you have determined the BC, measuring and accounting for atmospheric pressure, wind speed and MV changes that alter trajectory and drift significantly at LR is necessary for first round hit capability, with very high confidence.
This can be easy duty when using a a Palm computer with ballistic program in the field, along with a method to monitor wind and atmospheric pressure, such as a Kestrel
4000 or Sherpa.
You must already understand where your MV will be at different temps and states of fouling, and if not, testing loads at different cartridge temps using a cooler, ice and thermometer to stabilize ammo temp for testing throughout a range session can provide the info very quickly.
Actual drop determination -
You must record MV, altitude and station pressure then correct it to sea level for the BP during this testing procedure in order to create a baseline condition from which to work from in the future, without this, results are virtually meaningless at a later date.
Use the BC you have determined to be accurate, along with a drop chart that will keep you on paper out to 1000 yards.
Aiming at the top edge of a 8' piece of plywood beginning at 300 yards on out to 600, with a very perfect 100 yd zero to start with will allow you to measure "actual" bullet drop with a tape measure. Make no turret adjustments; just let the bullets drop at each 100 yard increment on to 600 with 3-5 shot groups. I use freezer paper to cover the plywood with, and keep the horizontal crosshair on the top edge of the plywood.
Record these drops, mark each bullet hole with a marker so not to be confused on subsequent shots, then go back and dial your come up for 600 yards and reshoot at bulls eye or target or some sort to verify.
Now move on out to 700, 800, 900, and 1000 yards dialing and recording your MOA dialed in, and then the groups point of impact (POI) difference from point of aim (POA) that you held.
You'll take these notes and record them as your baseline sight in conditions for "this" load.
You can get the atmospheric pressure at sea level (BP) from here http://www.srh.noaa.gov/data/forecas...70&city=Palmer
or somewhere else, but this is not pressure at altitude (station pressure), it's barometric pressure, which is pressure corrected to sea level, which your BC is based on also. Therefore, you must also know the altitude where the testing is happening if using BP to determine station pressure during testing.
If you have a means to measure station pressure at your location, you must know the altitude you are at to convert this to calculate the BP.
If you are at sea level shooting and there are very overcast conditions, you could be shooting in conditions with a density altitude of well over +5000 ft ASL, thus POI hitting MUCH higher than you would the very next day on a clear sky with a very possible density altitude of -5000 ft ASL, and hitting WAY low now.
Trajectories at standard conditions work at standard conditions, and density altitude can swing wildly so, account for it. The Kestrel 4000 works wonderfully for this, and I am sure a few others do as well.
You should also shoot at varying temps and pressures to verify how well your ballistic program is modifying your firing solutions at LR, verses what you are actually determining is needed. Once you get the BC and MV's all nailed down, most programs can do a great job of correcting for air density.