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...
Well, a "Cosplayer" on their way to an event has realy stirred things up...
www.dailymail.co.uk/news/article-14349311/amp/moment-man-sniper-rifle-Elizabeth-line.html
Unless you are dead set on an AEG, the WE Apache GBB is a nice bit of kit. All the appropriate bits are metal and three round burst is fun. Kicks a bit though and mags are heavy and limited rounds.