Like Max said - you could create a bullet sprite the same width as your beam, facing in the same direction, and just make it, say, 200 pixels long. Then you could give it the Bullet behaviour, make it Instant Hit, and set the speed to 200.
Instant Hit just keeps looping in the same tick until it hits something, so it'd strike the target immediately. If it traveled 10,000 pixels at the size and speed I said above, I guess that'd be, what, 50 loops? Should be no problem for the CPU.