ATP: Relative Energy / Minimum Engagement Distance Calculator

Airsoft Trajectory Project

Section X-B: Relative Energy / Minimum Engagement Distance Calculator

Using the equations in Section 01-C: Kinetic Energy, it is possible to estimate the muzzle velocity of a given rifle for various BB masses.  For instance, if a shooter knows that his or her rifle fires 0.20g BB's at 400 feet per second, he or she can expect to have a muzzle velocity of 358 fps when using 0.25g BB's, or 327 fps when using 0.30g BB's.  The table below can be used to estimate the muzzle velocity that a shooter can expect when using different BB masses. 

Keep in mind, however, that this is an estimate.  As was explained in Section VI-A:  Determining Muzzle Energy, heavier projectiles spend more time transiting the length of the barrel, leading to a greater overall energy transferred to the projectile (to a certain extent).  In other words, using heavier BB's generally leads to higher muzzle energies.  For instance, a rifle that chrono's at 500 fps with 0.20g BB's will, at the very least, chrono at 408 fps with 0.30g BB's.  Since the 0.30g BB spends more time transiting the barrel, it is able to absorb more energy; consequently a rifle that chrono's at 500 fps with 0.20g BB's might chrono much higher than 408 fps with 0.30g BB's.   

This effect is scarcely noticeable for rifles firing at less than 1.5 Joules (or 400 fps with 0.20g BB's).  The calculator is reliable when working with <1.5 J rifles.  However, for high-powered rifles, the calculator should only be used as a rough estimate of the muzzle velocity, and the shooter should expect a higher muzzle velocity than that listed.  Whether it will be 1 fps above what is listed or 30 fps above is dependent upon many factors, but the calculator will give you a rough idea of muzzle velocity for heavier BB's.  If an airsoft field is checking rifles to ensure that they do not go above a certain energy limit, then the heaviest available BB should be used for determining energy. 

The recommended MED is an estimate of the minimum engagement distance that a shooter should use such that the given BB mass fired at the given muzzle energy will have dissipated enough energy to ensure a maximum impact energy of 1.00 Joules.  As explained in Section VI-C:  Recommended Universal MED's, for equal muzzle energies, heavier BB's will dissipate energy at a much slower rate and consequently require higher MED's.  This is reflected in the MED chart below.  For instance, at 400 fps, a 0.20g BB has an MED of 20 feet, whereas a 0.25g BB has an MED of 25 feet.  This is because a 0.20g BB fired at 400 fps will have slowed to 328 fps at 20 feet (the impact energy equivalent of 1.00 J), whereas a 0.25g BB fired at 358 fps will require 25 feet for it to slow down to 293 fps (the impact energy equivalent of 1.00 J with a 0.25g BB).

It should be noted that the recommended MED is calculated using a logarithmic scale as opposed to the standard calculus equations used for the majority of the calculations performed in the Airsoft Trajectory Project.  This is done to make the calculations faster in HTML.  As of such, the Recommended MED's listed are only an estimate (whereas the MED's listed in Section VI-C:  Recommended Universal MED's are calculated fully and should be considered correct).  The MED's listed here are generally very accurate, but for high-powered shots (those greater than 3.0 Joules), the MED's listed may be off by 5 feet.  Fortunately, when the 5' error does show up, it is 5' above what is actually recommended (meaning that the calculations are still safe).

Projectile Mass:           Chronograph Results:

 grams            feet / second           meters / second