Paintball Pain… What Makes Paintball Impacts Hurt?
When asked about paintball impact pain, most people relate it to energy… but let’s take a closer look at kinetic energy and some other considerations. It is clear that some impact locations hurt and welt more than others, so for the purposes of this discussion we will assume that all impacts described herein are at the same body location.
It does not take long for newbies to learn that bouncers hurt more than balls that break, even though balls from the same bag which are shot at about the same velocity should have about the same kinetic energy (KE). General concepts of energy are easy to understand, but may be misleading. Not much has been published about some of the other factors that contribute to pain, and a broader understanding of impact dynamics may change your perception of Low Impact Paintball.
Let’s start with an important question: “Do two balls of equal mass and velocity hurt the same if they both break?” And my answer is: “No, not necessarily.” Some paintballs are tougher than others, and how they break can affect how much they hurt.
Consider paintballs that barely break. These tough paintballs hurt less than bouncers because they do ultimately break. However, they transfer more of their energy before they rupture, so they hurt more than they would have if they had ruptured immediately upon impact. Thru my many years of engineering consulting I have used “Tenacity” to describe solid materials such as rocks, which are reluctant to fracture. I also find the term appropriate to describe soft gelatin capsules which are tenacious, and do not break well. I hear more colorful descriptions on the paintball field.
KINETIC ENERGY DENSITY (KED)
Perhaps the most difficult impact concept involves Kinetic Energy (KE), and this sometimes leads to heated discussions between paintball experts. An understanding of how energy is transferred from a projectile to a target object (in this case, human) is important when considering velocity limits for the new ASTM Low Impact Paintball Standard.
For several years I have also been a member of the ASTM group responsible for projectile toy standards. During technical meetings for toy standards, I was introduced to the concept of kinetic energy density (KED). Simply put, if a projectile’s size or shape serves to concentrate an impact on a smaller area, it hurts more. It becomes even more complicated if the projectile changes shape during the impact. Soft gelatin capsules deform and their leading surface starts to flatten out during impact. This gives a larger impact area across which to distribute the impact. The larger the impact area, the less the pain.
Consider two paintballs of different diameters with the same KE. The smaller paintball is lighter in weight and must travel faster to achieve the same KE. Even if both paintballs deform, the smaller one has a smaller foot print, so the impact area is reduced. This is one reason why a light weight .50 caliber paintball normally hurts more than a heavy .68 caliber paintball having the same kinetic energy. The slower .68 caliber paintball has that same energy distributed over a larger contact area, resulting in a lower KED. Even measurements by various “impact meters” may not reflect the wallop of a paintball, as certain locations on the general impact surface may experience significantly greater forces. Any force, when distributed over a greater area, results in a decreased pressure.
Theory sometimes must yield to empirical results. Scientists must believe the net result, even when they cannot fully understand the theory. Years ago, when I was developing .55 caliber “Foamballs” to shoot out of low impact paintball markers, I enlisted the aid of a reusable paintball manufacturer, who provided me with prototype balls that did not bounce. These “dead” balls would drop and flop on the floor. This characteristic made them easy to gather up for re-shooting because they remained near the point of impact …but boy did those little suckers hurt. Although they did deform a bit upon impact, those little balls felt like they transferred every bit of their KE to my back during testing. However, KED, the best measure of a paintball “welt factor”, may not have been as high as a tough-skinned paintball which may hit with less deformation. Did you follow that? Lots of weasel words. Projectile deformation, Kinetic Energy Density, tensile and/or compressive shell failure, energy transfer mechanisms… Thinking about all of the physics and interacting material dynamics can make my hair hurt.
Nothing beats a paintball that survives the rigors of lock, load, and launch; and then is barely able to stay together in flight until it touches another surface. This is the holy grail for tournament paint, and is also the definition of a perfect paintball for low impact games. Perfect because it reduces the impact pain, and perfect because the smaller caliber paintballs will break more readily. Paint selection is very important for all reduced impact games, regardless of what size paintballs are used (from 11 mm to .68 caliber). Reduced impact game organizers (especially low impact games) should network, and they should communicate when bad paint hits the street or the seasons change.
I am fascinated by impact considerations, and I enjoyed reading about how the US military selected the .45 caliber round over the 9 mm Parabellum for a WWII sidearm. Momentum and “knock down” power were better for the .45 ACP Colt 1911 Navy, even though the Luger may have had a higher kinetic energy. But that’s another story, and a similar argument.