Found this in a comments section on the "How To Play Imports" post for the Dreamcast...
"anybody have any clue about using a jap controller (light gun) on a us console?
would installing a mod chip fix this?
theyre both ntsc right"
Any ideas?
Never tried it myself...
How To Use Japanese Light Gun on US System
I think the japanese light gun for the dreamcast works just fine on a US console. They real issue is that sega removed support for the japanese light gun from the US releases of HOTD2 and the other shooters (although I think Virtua Cop on the Smash pack still works with it). It'll probably work fine with the japanese release of HOTD2.