when you change the velocity the part will quickly decelerate due to ‘technical counter forces’. This means you may set the velocity to 5 but due to the parts mass it will decelerate to Velocity(0,0,0) very quickly (could be milliseconds).
You would need to make sure it has constant velocity, this could be done by creating a bodyforce or with a script.
Edit:
There will be more ways than that, was just listening the first two I thought of.
And also what @BenMactavsin said is a great point as if you also want it to be able to shoot behind as well then then you would want negative values as well.