Thread: Assessing resulting vertical spread from ES (or SD) for your load at distance

….. a worked example:

Let’s say a bullet is being used with an average muzzle velocity of 2800 ft/sec. The ES of the load in that particular rifle is known to be ± 10 ft/sec, and the interest is in the maximum amount of vertical spread this could end up producing on a target at 1 kilometre.

The Applied Ballistics or JBM balllistcs calculator is set up with all the necessary inputs (including the usual zero distance of say 200m) and the drop is found for a 2800 ft/sec muzzle velocity. Lets say it is exactly 10 metres. Then the MV is changed to 2790 ft/sec (and 2810ft/sec, just to confirm), and new POIs at those settings are calculated and found to be say 70mm above and below the nominal. The ES is expected then to produce a 140mm spread on the target.

In fact the true dispersion from that size of ES is likely to be around ±85mm as follows: The calculations are repeated, except this time the zero distance is moved back in the simulation to as close to the muzzle as possible (ideally at the muzzle, but 1 metre may be the closest that can be entered). Because of the short zero it is necessary to also reduce the sight height to zero so the rifle doesn’t end up being tipped at a steep angle in the simulation. This then simulates a barrel held perfectly horizontally and where the bullet drops away below this line. More importantly with these settings the barrel remains in exactly the same position for both the 2800 ft/sec and the 2790 ft/sec and 2810 ft/sec calculations, giving a ±85mm difference in POI..
In the first example where the zero was 200m this is not the case: the barrel is simulated to launch the bullets at very slightly different angles for each of the three muzzle velocities, having the effect to partially compensating for the difference in MV.

The error using these ballistics programs to find vertical spread from ES becomes more pronounced the further out the zero distance used. Take an extreme example where the zero entered into the ballistics program is right out at the target at 1 km. The simulation will clearly calculate no change in the POI for any variations in muzzle velocity !