[Pontiac]

Another major problem with a projectile weapon in space is that once fired the projectile is a danger until that projectile finally strikes another object and imbeds itself in that object.

A projectile fired from a spacecraft would continue on its trajectory for millions of miles without loss of momentum. Conceivably that projectile could slingshot around a planet and return to the point of origin destroying the spacecraft that launched it comic book style.

[Sherman Logan]

An excellent point. Space travel would have enough problems with space junk that was accidental in nature without adding projectiles flying all over the place.

[PeteB570]

Would it not be the same for the laser?

You fire a three second laser pulse and you have that 3 second length of laser beam continuing thru space until it hits something? Granted after a few light years it may be dissipated?