Not really, the weapon does not need to slow down. If its just a big enough chunk of rock that is aimed at a planet and it speeds up the whole way reaching some

Not really, the weapon does not need to slow down. If its just a big enough chunk of rock that is aimed at a planet and it speeds up the whole way reaching some impressive fraction of the speed of light before impact, the only hard part is the math to aim it. We are reasonably close now to being able to strap some engines on an chunk of rock and send it on its way randomly in one direction. We are very far away from being able to start a seed colony in another solar system.