I'm gonna bypass requoting your reply (learn to use quotes

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1. I don't think you follow.... USB has 4 wires, the guns have 5. This means the guns are NOT usb, at least not fully. The xbox uses some proprietary method to interface the guns, one that really isn't documented because, well there aren't many lightgun games for the xbox.
2. Google how a lightgun works sometime. The gun has to interface with the video signal, and by that I mean you must have a connector between the video out of the console/pc/whatever and the monitor. In other words, it has to be hacked into your vga connector to work on your arcade monitor. As sliver said, in the case of the arcade monitor it isn't going to be a huge deal, but it often requires a small, custom pcb and some fiddling. Even after you get it as good as it'll do, the act-labs guns will be noticably more accurate. Timing is the key for lightguns, so it's best to have one designed for the device in mind.
3. Read Silver's post