I realize you mean for the purpose of game balance, but the thing is, high spread for a minigun, or any rotary gun, for that matter, doesn't make sense at all. If anything, since each barrel is moving along a tangent vector as a round is fired, all of the rounds would move slightly in the direction of that vector. Effectively, the gun might shoot slightly to the left or right and down if the top barrel fires or up if the bottom barrel fires, but all the rounds would move in the same direction, not outward randomly. In any case, the effect on the accuracy of the gun would be negligible at its intended engagement range.
Edit: I realized something else. With a shorter barrel, the time spent under the tangent vector imparted by the barrel spin would be less, so the effect on the end trajectory would be less, as well. Although, the muzzle velocity would be reduced, too, since the propellant has less time in the barrel to impart energy to the bullet.
I'd like to add that in the case of a minigun accuracy doesn't matter given the amount of lead going downrange and that you're typically using it against a large target like a vehicle or house.
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Edit: I realized something else. With a shorter barrel, the time spent under the tangent vector imparted by the barrel spin would be less, so the effect on the end trajectory would be less, as well. Although, the muzzle velocity would be reduced, too, since the propellant has less time in the barrel to impart energy to the bullet.