Well there are theoretical formulas, but they never work in real situations. The problem is, a BB can come out of the barrel with a certain muzzle energy with one type of BB (0.5*m*v^2). If you then used a heavier BB, it should theoretically be the exact same muzzle energy. However, heavier BBs tend to have a larger amount of energy imparted on them by the system, and will therefore come out with a higher muzzle energy.
Your best bet is to do the theoretical calculations ( (0.2)*(360)^2 = (0.28)*v^2 solve for v), and then assume that it is going to be a bit more than that.
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