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The "transition zone" from interior to exterior ballistics in hadguns calibers is very small (2"-4"?), and added acceleration almost nill. This can be found calculating the forces acting on the bullet at the moment it exits: gas pressure times bullet diameter is higher at the back than aerodinamic drag calculated for muzzle velocity, and this drag is not even real (it's very difficult to calculate) because of the forward gas flow around the bullet (the gas moves faster than the bullet). After leaving the muzzle the pressure drops so fast it can not accelerate the bullet. When you have a much larger gas flow (rifle magnum calibers, 50 BMG, artillery, etc.) this distance increases, and this gas flow can make a few fps increase. If you have a muzzle brake you are redirecting this flow and losing this few fps.
bfoster is right about bullets slowing down VERY slowly inside the barrel, and I've read articles by Bob Forker and other technical gunwriters reaching the same results from experiments.
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