PAINTBALL IMPACT DEFINITION
… “kinetic energy” is sometimes used improperly in definitions and calculations. Paintballs are often described as “frangible projectiles”. However, paintballs that do not break should not be classed as frangible. And if they do break, the relative impact is only a partial value of the theoretical kinetic energy, and is difficult to calculate. Another confusion is how the KE is distributed, or transferred, throughout the impact process. Kinetic energy density might be a fair comparative measure of impacts, and accounts for some of the additional variables such as projectile deformation, which occurs during most impacts even when the paintball does not break. Deformation is important, as the kinetic energy might be transferred more slowly and across a greater surface area. For example, consider two paintballs that are of equal weight and shot at the same velocity, but neither ball breaks. The more rigid ball (having the same kinetic energy) will hurt more than the paintball which deforms more. This may seem obvious to anyone who has felt the impact of a frozen paintball. It also makes sense when considering certain specialty rounds that are used in paintball games.
As described in a previous blog post, certain paintball impacts where the paintball actually breaks can hurt almost the same as bouncers. “Tenacity” is a mysterious characteristic which relates to the relative “frangibility” of paintballs which break. I consider paintballs which are reluctant to break to be less frangible (or more tenacious) than other paintballs which happen to break easily. So, how should we approach ASTM standards for reduced impact levels of paintball? This leads to some interesting questions:
- Is kinetic energy a fair measure of impact for paintballs that break?
The answer is “NO”. Kinetic energy is the maximum energy available for transfer from a paintball that does NOT break.
- Is kinetic energy the proper measure for paintballs that do not break?
Again, the answer is “NO”. Kinetic energy DENSITY is a fair measure, but a precise calculation is not in my pay grade. However, available empirical data is logical, and indicate much different results than would be predicted by most engineers.
- When we set limits for ASTM standards, should we be considering impact energy for breakers, or for bouncers?
In my opinion, bouncers illustrate the worst case, and are easier to correlate to a measure of pain or trauma. However, kinetic energy density, rather than overall kinetic energy, is the most fair way I know to consider the impact of any paintball. Some industries have previously established KED threshold values for certain types of injuries, including eye trauma such as retinal detachment or severed optic nerve. Most theoretical energy values are poor approximations of the actual energy transferred upon impact with human tissue.
Kinetic energy density may be correlated to pain or trauma, but the physics go deeper… literally. When considering bouncers, there are additional factors that make some bouncers hurt more or differently than other bouncers. I cannot begin to provide theoretical calculations which reliably predict which bouncers hurt the worst, but experienced paintballers often have memories of hand or butt shots which were particularly painful… even when they broke. We will explore these notions in future blog posts.
Perhaps the best lesson here is to consider more than just kinetic energy when establishing firepower restrictions for paintball standards. Remember, paintballs which break easily have the same calculated kinetic energy as paintballs that are frozen. Pain levels and trauma are greatly affected by the dynamics of energy transfer. Frozen paintballs are prohibited at regulated play sites, but their kinetic energy is generally accepted as the basis for safe impact values. Where should we go for fair comparisons of reasonable paintball impact? The answers are finally floating to the surface as we finalize the ASTM standards for reduced impact paintball. For updated information on Low Impact Paintball, tune in to: http://LowImpactPaintball.org