Athyl, would be clearer if you stated what's the specific problem?
Body.IsBullet = true;
Body.IsSensor = true;
For the bullets body properties. Bullet for fast moving bodies.
When launching a bullet from e.g. a spaceship, get the direction of the Body who is shooting by:
Vector2 directionVector = ownerEntity.Body.GetWorldVector(new Vector2(0, -1));
Vector2 forceVector = directionVector * BulletForce;
The first line gets a world unit vector related to the direction of the ship (local vector is one unit upwards).
Second line creates a force vector, multiply the unit vector with the force to get a directional force.
Then on third line, apply a the force as an Impulse to the bullets body, it gets to the max velocity immediately.
in the above code "ownerEntity" is the players spaceship which is a Sprite. (Spaceship owns a Weapon which generates Bullets.)