I knew a guy whose upgraded Thompson had a legendary rate of fire, but was problematical because the magazine springs couldn't feed fast enough to keep up.
The days when you just put a 9.6v and a 1j spring in a marui and ran it till the gearbox exploded...
Once the BB leaves the barrel it will start to decelerate. There may be some slight residual acceleration from gas/air exiting the barrel but that would be negligible and only for a very short distance.
Ye cannae break the laws of physics...