It's possible but it just results in network desyncs.
If you think about it, you realize it can't work. The same projectile can not be going two different speeds for different players, because it would result in the same projectile being in two different places at once, which doesn't make sense. The result is the client sees something that is different than what it actually is on the server.
For example, you may see a rocket heading straight towards you, but you see it in slow motion so it's 10 feet away, and you have plenty of time to move out of the way. Then you suddenly blow up because the server says its right in your face.