You got it, but it would have very high damage. Think of it like a beam that does exactly the same thing as the Trebuchet, lob two off in single player at a fighter ~4000M away and it takes that fighter out in one shot. After all, what fun is a beam that doesnt punch through shields?

It would have to consume a lot of ammo space for whatever powers it (plasma cores?), because with a one hit kill weapon you want it to consume a lot of ammo so you can only get 2-4 shots tops before needing a reload.
I think field reloads would be fine, you could make a shorter range version like your idea, you lock on just like you would with a aspect seeking missile, but instead of a missile you get a beam that fires directly at the target even if you arent pointed directly at the target. The beam would also be very useful if you needed to quickly destroy a turret on some ship thats a long ways away.
If you want to weaken it even more you could use the existing "miss factor" in beams to make it miss sometimes but I think that would take away from the fun, after all aspect seaking missiles never miss a big ship.
Want to explain how the targeting system works? Simple, anyone know how a CRT computer monitor works? A capacitor powers an electromagnet that bends the beam from the electron guns at the back of the monitor so that the beam points to the correct pixel on the screen and lights it up. You could say that the fighter beams are particle beams and that the targeting system uses powerful magnets to bend the beam so it is pointed at the target.
-Krom