I'm looking for a formula that can be easily calculated in the field that only requires a minimum of data:

Muzzle velocity (from your rifle and your load)
Target range in yards
Wind speed
Wind direction using the 'Clock Method'
Zero range in yards

The formula needs to correct for loss of velocity over distance. It would only be used out to about 500 yards, so humidity and rotation of the earth don't need to be calculated in as well as altitude and temperature as I will rezero my rifle in the area and conditions I will be shooting.
I loke the Point Blank program but feel if I had this formula I could make a more accurate range card, and I am also making a program that usilizes it to run on my TI-86 graphing calculator. I already have a couple useful simple shooting programs on there I made.
I am planning on buying the Sierra reloading manual but not for a while.

You don't have enough information listed to calulate bullet drop or trajectory. The altitude is important because there is less resistance to the flight of your bullet at higher altitudes than at sea level. The ballistic coefficient is vital to the calculation of time of flight which is necessary to give you the amount of bullet drop per unit of distance. Altitude gives you the basic air density that is also used to calculate wind drift.
It is easier to carry a ballistics print out from your minimum range to the maximum range with a range of wind drifts calculated.
A ballistics program, the one that I wrote, uses recursive calculations, to compute all of the data for each yard or meter from the muzzle to the maximum range. The more calculations that are made the more accurate the results are. To do this in the field would require a ballistics program where you can enter the variables at the beginning of the calculation. You could hard wire your bullet information if you always use the same bullet and your muzzle velocity if you always use the same load but you would have to add: angle of target from level, altitude and temperature, wind speed and direction, range to target (zero would be constant unless you wanted to make changes to your sights in the field).
All of that still ignores the effect that the bullets spin has on it's path in relation to side winds. (a bullet with a right hand twist will tend to drop faster if there is a wind from the right and drop slower if the wind is from the left) and the spiral flight of a bullet. (most bullets travel in a spiral that increases from the muzzle to some point down range and then it diminishes in spiralling to the maximum range.) That is why we have to sight our guns in when conditions or range changes.
I don't carry a trajectory table when I hunt because I have my sights set to a point-blank range of 200 yards. That is the maximum range that my bullet can be expected to hit within 4 inches of my point of aim under the most severe conditions that I will attempt a shot. If it is too windy, or raining, the angle is too steep I don't shoot. At 200 yards under ideal conditions I can keep my shots in a one to one and one-half inch group. But while hunting I may be at 2500 feet or 7500 feet, the temperature might be 65F or -10F, I migt be shooting level or up/down grade at 30 degree angles. It might be calm or have up to ten mph winds. The humidity is usually low but it can be very high too. What that all adds up to is that each shot is unpredictable to some extent. I want to make sure that my shot hits and cleanly kills the animal. So I allow for those vaiables in calculating just how far I can reasonably expect my shot to remain no more than four inches from my point of aim.

Its extremely complicated to calculate drop, there is not a "simple" formula to calculate this. Your best bet is to get a laptop and use any ballistics program available to calculate it.

Drift however can be calculated by knowing the time of flight it takes your bullet to arrive at its distance. If for example the crosswind is blowing 90 degrees to your line of sight at 10 mph, that equals 14.667 feet per second. So if your bullet takes .25 second to reach its target, .25 * 14.667 = 3.667 ft of drift. That begs the question however, of how you are measuring the time of flight, something calculated in with a ballistics program.

Seriously, its really complicated with alot of factors you arent concidering... just get a laptop and drag it to the range


-DallanC

Lol, thanks. I'm just tryin to see if there's a 'simple' formula out there for a person who fiures the same gun with the same bullet around the same altitude and conditions under 500 yards out there. Guess not, thanks for the assist anyways.

[84jeepj10]I'm looking for a formula that can be easily calculated in the field that only requires a minimum of data:but not for a while. [/quote]


There is no "easy" formula. In fact, what you want is pretty complicatated.

As Dallan has pointed out the formula for "drift" is easy enough, on a computer and with a known time of flight, but there is more to calculating a bullets path than drift; "deflection" is more important. That concerns the fact that the bullet is not only drifted by wind but it's actual heading is also changed. At the speeds a bullet travels, moving air has an effect much like hitting a moving object so the flight path is directed down wind, deflected in other words.

Even for those programs that accurately do the math and print tables, it will be rare for them to actually predict a wind blown bullets path in the world much better than giving a general indication. Cross winds vary both in strenght and direction over a 100 yards, it's 5x as much over the range you mention. The calculations must assume you accurately know the wind speed, that it's constant over the distance AND that you can accurately gauge that distance at long ranges, none of which is likely to be absolutely true.

Trying to predict wind makes it very hard to make long shots, wind is much more changable than gravity! With much wind, hitting at a distrance is more luck than accurate calculations or high velocity.

84jeepj10 wrote:

Lol, thanks. I'm just tryin to see if there's a 'simple' formula out there for a person who fiures the same gun with the same bullet around the same altitude and conditions under 500 yards out there. Guess not, thanks for the assist anyways.

Post date is old but if you feed me some data I can get you a drop and drift chart...if yer still interested.