Paintball guns are pneumatically powered (air powered) devices that allow a compressed gas to expand behind a paintball which propels the paintball down the barrel of the gun. As the gas expands, the paintball accelerates down and out of the barrel. Four factors affect how fast the paintball leaves the barrel: the speed that the gas expands, the volume of gas expanding, the weight of the paintball and the fit of the paintball in the barrel.
Different gases expand at different rates. For paintball guns, carbon dioxide (CO2) and compressed air are generally the only two gases used. For the purpose of this article, they both expand at a very similar rate.
What does vary, though, is the volume of air that is used to shoot the paintballs. Compressed air guns use regulators to determine the pressure of the air that is released, with higher pressures indicating a larger amount of air being released into the firing chamber. CO2 doesn’t use a regulator, but the time that the valve is open will similarly determine how much of the gas is released (note, though, that since CO2 is stored as a liquid, sometimes a liquid is actually released into the gun). Generally, the larger the volume of air, the higher the velocity of the paintball.
Another variable is the weight of the paintball. Heavier paintballs require more force to move, which means that all else equal, heavier paintballs will leave the paintball barrel at a slower speed. To shoot heavier paintballs at the same speed as later paintballs you must increase the pressure behind the paintball (such as by turning up your regulator or increasing your spring pressure which will keep your valve open longer).
The last variable that determines how fast a paintball will shoot is also the most important when it comes to shooting marbles: how well the paintball fits into the barrel. A paintball is a globe-shaped object that fits into a cylindrical barrel. As gas expands behind the paintball, it can either slip around the edge of the paintball (in the gap between the paintball and the barrel) or it can push the paintball down the barrel. When there is a good paint-to-barrel match, a paintball will just barely fit into the barrel and won’t roll down by gravity alone, but will go down the barrel with a small puff of air. Paintballs that are slightly smaller (a few thousands of an inch) than the barrel, though, will still shoot because as the air pushes the paintball the pall will deform slightly, similar to when you push on opposite sides of a balloon, until it is large enough until it fills the barrel and creates a good fit. Excessive deformation of the paintball will hurt accuracy, but the ball will leave the barrel at the same speed as a paintball with a good initial paint-to-barrel match.
Marbles can be shot by paintball guns, using the same principles that are used to shoot paintballs. The key is that you have enough air behind the marble to push it out the barrel. First, the marble must be nearly the size of the barrel. Paintball barrels are .68 caliber meaning you need to find a marble that is that size or just a bit smaller. A marble larger than that will not fit into the barrel and a marble more than a little bit smaller will not block very much air. While paintballs will deform a bit to fill the barrel, marbles will not. The result is that a marble will be shot, but a lot of air will escape around the marble and the barrel and the marble will not be shot very fast. To shoot a marble, then, you need to find a marble that will fit into the barrel of your gun, but will just barely fit into the barrel.
Beyond simply shooting the marble, if you want to get any velocity from the marble, you may have to increase the velocity of your gun. Marbles may be heavier than paintballs which will require you to boost the pressure on your gun.
Why You Shouldn’t Shoot Marbles
There are a number of reasons why you shouldn’t shoot a marble with a paintball gun including that it could be dangerous, it could hurt your gun and it’s really not that cool.
The primary reason you shouldn’t shoot a marble is because it is dangerous. A paintball is designed to break apart and cause minimal injury on contact. Marbles, on the other hand, are not intended to break. A marble shot from a paintball gun could cause severe injury to a player and could potentially break through a mask and blind someone. For this reason, you should never shoot a marble at another player. In fact, when a marble is shot through a paintball gun the paintball gun is now a weapon and the same safety steps used with firearms need to be followed.
A second reason why paintball guns and marbles don’t go together is that paintball guns are not designed to shoot them. The result is that you can actually damage your gun from firing marbles. The hardness and weight of marbles may damage your gun, particularly the barrel and the bolt, and the excessive pressure needed to fire it will sometimes cause other harms such as blown o-rings and over-stretching of springs.
The last reason, which really shouldn’t matter to most people, but probably does, is that shooting marbles is not that cool. A marble that shoots from a paintball gun is similar to shooting a BB or pellet gun. The difference, though, is that they generally go slower and are less accurate. It’s a hassle to find marbles that fit into your gun and they cost a lot more than paintballs. The best thing to do, then, is to use a paintball gun for what it was intended to do: namely, to shoot paintballs.