I find it depends more on where the weight of the gun is than the length of the barrel, if the gun is muzzle heavy then I find I tend to be less stable and shoot low, if the gun is butt heavy, I tend to shake a bit and aim too high, I like a nice evenly weighted gun. All that said I think you would be well off with a 27" barrel, any shorter and you might find it a bit too loud...

M82A1