Spudgun accuracy

Accuracy is a measure of how consistently a gun shoots; an accurate gun shoots in the same position relative to where the gun was aimed.

The various causes of inaccuracy in spudguns are probably the same as those in firearms, though are of unknown importance:

  1. The flexing of the barrel, which results in the projectile leaving in the wrong direction or velocity.
  2. The movement of the launcher under recoil.
  3. The tumbling and resulting aerodynamic lift forces on the projectile.
  4. In rifled barrels, the spin throwing the projectile off of the intended path due to the center-of-mass not being in the center of the projectile.
  5. The muzzle blast deflecting the projectile.

The first problem can be minimized by stiffening the barrel; this can be accomplished one of two ways:

  1. By shortening the barrel.
  2. By stiffening the barrel - either by using metal pipe, using larger diameter pipe, sleeving the barrel, or using some sort of reenforcement.

The second effect cannot be eliminated, but it's negative effects can be mitigated by allowing the launcher to recoil the same on every shot.

The third effect seems to be one of the main ones, as spinning the projectiles reduced group sizes by about 50%. Alternative fixes exist, including:

  • Using round munitions, which cannot generate lift unless they are spinning. However, round ammo frequently will exit the barrel with a spin. Unfortunately, the spin is frequently highly variable from shot to shot. Sometimes the round projectile will have back spin which makes the round sink faster, or forward spin which makes it rise, or side spin which makes it curve. If the round can be given a consistent spin then at least the rise/fall/curvature will be consistent from shot to shot and give increased accuracy. One method to give a round projectile a consistent spin is to incorporate a "hop-up" near the muzzle, as is frequently done on paintball guns.
  • Fin-stabilizing the ammunition.

The fourth effect is probably not very significant, even given the large variations in the location of the center of mass in cut potatoes, but can be reduced in severity by using as low a twist rate as possible as well as cutting the potatoes such that their COM is inline with the center of the bore.

The fifth effect can be reduced by porting the barrel and/or crowning the barrel. Crowning means the inside lip of the muzzle has been counter sunk slightly and that the lip is uniform around the circumference of the barrel. A well cut double-beveled spud cutter muzzle would be a good crown.