How do I build a laser/IR target?
The 'Laser' target systems that I know use IR for hitting the target. The gun is a variation on the TV remote, with a lense to focus the IR. Yes, you can focus IR just fine. Or use a LED that has a narrow beam by itself. The target uses an IR detector module, like a TSOP1738 or a newer variant.
The laser is only for the spectacle. (Indoors they use mist machines to make the laser vissible, outdoor systems often don't use a laser at all).
A this very moment 10 student groups are developing the firmware for 'laser' tags for me for a course in 'embedded systems programming'.
For a suggestion on what to implement you can read any website that talks about sending and/or receiving IR control signals. No need to use a specific one, as you make both the sender and the receiver.

Fun to re-read this a few years later. The target chip changed from LPC2148 to LPC1114 and now to SAM3X8E (Arduino Due), but the course assignemnt hasn't changed a bit.