What makes On Target Unique?
Rifle Geometry
While many trajectory tables and other tools have either a predefined difference between LOS and bore line (tables are typically done for one such Scope Height), we are not aware of any that use this for other than error from LOS calculations. On Target uses this information to calculate the angle between the plane of the bore and the plane of LOS when a Sight in Range is defined. While very small (0.0526054 degrees for John’s BAR on the Preview page), this angle determines the velocity component in the vertical direction. Since those that deal in “trajectory” solutions utilize the well known G1 model for calculations, this angle is never known.
The Barrel length for the rifles you define is a part of the Rifle Definition. This allow On Target to calculate the actual muzzle velocity for the rifle from cartridge data which has its own barrel length used by the manufacturer in specifying the velocity.
On Target is not “bullet” based…it is Rifle based. You must define the geometry of your rifle in order to understand how it will perform. While obviously exaggerated, the diagram below clearly shows how this angle is affected by both scope height and sight in range.
The Model
For years, the G1, and sometimes G5 and G7, tables have been used to define the trajectory of a bullet. In fact, published Ballistic Coefficients are based on the G1 model. Some manufacturers will provide data for their bullets giving the G1 based Ballistic Coefficient for different velocities. On Target utilizes a proprietary model that calculates bullet dynamics every 100 microseconds of simulated flight. The model incorporates factors that continuously compute aerodynamic forces as a function of atmospheric conditions and velocity. The result is a continuously adapted Ballistic Coefficient.
The use of this sophisticated model has two profound results:

Precision is not compromised.

Don’t try to run on an old 486! Not enough MIPS.
The number of factors that will affect a bullets trajectory are large. They include buoyancy, gravitational changes due to latitude and altitude, coriolis changes due to latitude, humidity, air density, temperature, etc. Any factor of significance is incorporated in On Target’s unique model.
Scope Adjustments
Since On Target computes the angle between the plane of the bore and the plane of LOS, and uses this as the basis for all calculations, changing this angle by adjusting the MOA (clicks) on the scope allows On Target to calculate the impact of such changes. Actually, when a shooting objective is defined (say, deadon at 400 yards or perhaps a maximum error of 3 inches), On Target will increment/decrement the MOA using the precision of your scope and run its model until the objective is achieved. The equations used in this sophisticated model do not lend themselves to closed form solution, thus each shooting objective can result in millions of calculations being performed to determine a unique solution. Fortunately, PC’s can accommodate this need with ease today. Ten years ago during the early development of On Target’s model, engineers would wait 1015 minutes for a result. Today, this is measured in fractions of a second.
The linear change at a given range for one MOA change in your scope is a function of the basic geometry of the rifle, the cartridge being fired, the BC of the bullet and atmospheric conditions. It is not a constant, though some assume it is. Change your scope clicks and the velocity in the vertical direction changes, changing the velocity in the horizontal direction. The bullet arrives at a fixed location after a different TOF (time of flight), and with a different rate of change in the vertical plane. So, how many clicks per inch…the model must shoot under these new conditions to find out!
Chronograph Distance
While cartridge manufacturers have done an outstanding job of producing cartridges with minimal variation in either BC or muzzle velocity, these variations are not zero. Published muzzle velocities are also based on a “typical” rifle, as they must be. While fine for most shooting objectives, precision long distance shooting requires that the muzzle velocity be accurately determined for a cartridge/rifle pair.
I have been told that there are two kinds of people that have chronographs: those that have shot them and those that have not used them. For that reason, we tend to place our chrono close to the muzzle. However, muzzle blast will cause both damage and erratic reading from the chrono, so we adopt something between 1030 feet as a reasonable distance.
Assume we are at the 10 yard point. What we are really measuring is the velocity in the plane of the chronograph, not the total velocity, and we are measuring it after the bullet has slowed down for 10 yards. While the former error is insignificant except in the cases of obscure scope heights, the latter can…and does…introduce errors ranging from 0.5% to 1.5%. Although small, these errors will compound should one attempt to use, say G1, to determine a BC. On Target utilizes your defined distance to the chrono and rifle geometry to calculate what the velocity would have been at the muzzle…not 10 yards away! This seemingly minor error can result in a totally missed target at long ranges if not considered in the computations.
Calculation of Scope Corrections from Target Data
By use of its sophisticated model, and knowledge of your rifle geometry, On Target is able to determine the precise scope adjustments required to move your point of impact (POI) to the bullseye! It does not store any “standard” for the affect of scope changes at a given range. This is calculated by On Target for your rifle, cartridge and conditions.
A statistical analysis is performed on your entered shot pattern. If On Target detects shots outside a statistical limit, it will identify them, and ask the shooter if they want to eliminate those shots from the resultant corrections computations. It will then utilize its model, making adjustments equal to your scope resolution, until errors from deadon are minimized.