Section VI:  Minimum Engagement Distance

        Section VI-A:  Determining Muzzle Energy

Provided that the mass of the BB is known and the muzzle velocity can be determined, muzzle energy can be easily calculated using the formulas as outlined in Section I-C: Energy.  For most airsoft rifles and pistols, the muzzle energy for a 0.20g BB will be the same as for that of a heavier BB.  As muzzle energies surpass 1.50 Joules, however, the muzzle energy correlation is not always equal.  In high-energy guns, it is possible for the BB to accelerate and leave the barrel before the piston has had a chance to reach maximum compression.  In this case, pressure will still be building as the BB exits the barrel; consequently the BB will not be able to fully absorb the energy from the piston.  When a heavier mass BB is used in this gun, the BB will accelerate at a slower rate and remain in the barrel longer, thereby absorbing more energy. 

An example to this would be a sniper rifle (TM PSG-1) recently tested:

BB Mass 0.20g 0.25g 0.30g 0.43g 0.88g
Muzzle Velocity (fps) 597.4 551.2 519.3 436.9 305.2
Muzzle Energy (J) 3.32 3.53 3.76 3.81 3.81

This is important to realize if guns are tested prior to events, as someone could arrive with a sniper rifle shooting near 600 fps with 0.20g and be given an MED based upon that muzzle energy, assuming that if the shooter was using 0.30 BB's, the rifle would be shooting at around 488 fps.  As observed above, in reality the rifle might be shooting as much as 30 fps (and 0.40 J) higher than estimated.  Consequently, a good recommendation is that a rifle be tested using the BB's that the shooter plans to use in the match as well as 0.20g. 

 

        Section VI-B:  Safe Impact Energy

It is often stated that a 6mm projectile can pierce skin if the impact energy is greater than 1.35 J.  Though I have not personally seen the document stating this, it sounds entirely plausible. Of course, there are other factors in play here, such as the angle at which the impact happens (straight-on vs. a glancing shot), as well as the place on the body where the impact occurs (as skin thickness and strength varies over one's body).  What doesn't sound right is when I read someone stating that it is actually an energy of something (such as 4.00 Joules) and that this has been determined by "leading experts" and applies to all objects.  It is the caveat that the listed energy "applies to objects of any diameter" that is clearly an errant claim.   People forget to take into account the object's size when they are trying to determine what constitutes a safe impact energy.  A projectile's size is important because it determines over how much area the impact will be distributed. 

Simply stated, objects with a smaller diameter will be able to pierce skin at a given impact energy more easily than that of a a larger-diameter projectile.  An easy way to think about it is to take two objects of different diameter (size) and consider the impact by each at equal energies. 

A 0.20g, 6mm BB moving at 656 fps (or 4.00 Joules) will easily pierce skin.  By comparison, a regulation golf ball (1.62 ounces, or 46g) moving at 75 fps (which is around 50 mph) will have an impact energy of about 12 Joules.  However, a golf ball moving at 50 mph will not leave a bruise, much less pierce skin. 

The reason for which the 6mm BB will pierce skin yet the 42mm golf ball will not is that the golf ball's impact energy is distributed over a much larger surface area.  For the 6mm BB, the energy is distributed over a frontal area of about one-twentieth of a square inch, whereas the golf ball's energy is distributed over a frontal area of over two square inches.  So when we discuss impact energies, it is important to realize that impact energy alone will not tell us whether or not the impact is safe, we must consider both impact energy and impact area. 

One could argue that the golf ball would not penetrate due to the lower speed, however, again it is important to stress that it is the impact energy relative to the impact area that causes penetration; and not just impact energy alone.  Since all 6mm BB's have nearly the same orthogonal area (i.e., impact area), we can focus on impact energy by itself since area is taken as a constant. 

While that may seem like superfluous information, it is important to recognize that due to the extra surface area and consequent distribution of impact energy, 8mm BB's are inherently safer than 6mm BB's.  For designated snipers firing 6mm projectiles, I prefer a minimum engagement distance based upon an impact energy of 1.00 Joule.  Because a 8mm projectile distributes the impact energy of a significantly greater area (as the cross-sectional area nearly twice as large), a higher impact energy could probably be used for determining MED's, however without any supporting data, I felt that it safer recommendation was to use the 1.00 Joule limit for 8mm BB's as well.   

To get below 1.00 Joules, a 0.20g BB would need to moving at less than 328 fps.  Each BB weight class has a specific maximum impact velocity:

 

  BB Weight Class (g) Max Impact Velocity (fps) Max Impact Velocity (m/s)
  0.20 328 100
  0.25 293 89
  0.30 268 82
  0.36 244 75
  0.43 224 68
  0.34 252 77
  0.45 219 67

 

Keep in mind that a safe impact energy is independent of size and only dependent upon mass and velocity; a 0.34g 6mm BB will have the same safe maximum impact velocity as a 0.34g 8mm BB (in this case, 252 fps), if the maximum impact velocity of each is to be taken as 1.00 Joule..

 

 

All text, images, and linked webpages are the property of cybersloth.org and may not be reproduced without permission.  For more information, contact airsoft@cybersloth.org.

 

The Airsoft Trajectory Project

Copyright 2005-2006 cybersloth.org  -  All rights reserved