Size:
The combustion of the fuel in the firing chamber will create a set pressure that has to escape through the barrel of the spudgun accelerating the potato out at a specific velocity. The size of spudgun barrels usually range from 1.5 to 2.0 inches in diameter with 1.5 inches being the standard. The larger the diameter the less the acceleration of the spud projectile. This is due to Newton’s 2nd law F=ma which can be written as a=F/m or m=F/a. If the diameter of the barrel is increased the force acting on the spud is dispersed over a larger surface area and therefore it will have a slower acceleration and intern a lower muzzle velocity. Also with the larger diameter there is a larger projectile which will have a larger mass. This larger mass will reduce the acceleration of the spud as well. The length of the barrel is entirely up to you but remember the longer the barrel the more friction has to be overcome. It is safe to say that to an extent a longer barrel is more accurate than a short one. The typical range of barrel length is between two feet and four feet.
Rifling:
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The inside surface of a barrel can be riffled or unrifled. Riffling is essentially several spirals cut into the inside of the barrel to cause the projectile to spin in the barrel. When the projectile leaves the barrel it spirals down range much like a football. When the projectile spins on only one axis, the one parallel to the axis of the barrel, it reduces the drag on the projectile. The less the drag is the farther the spud will travel and with the reduction of cross forces, due to the wobbling of the spud, a higher accuracy can be achieved. The unrifled barrel does not provide this balancing to the flight of the projectile and it therefore is unstable and less accurate as a result. However due to the difficulty of machining riffling on the inside of a 1.5 inch ABS pipe the unrifled barrel is the most common in use. |
