Sega of America canceled the North American release of the official light gun due to the Columbine tragedy. NTSC-U light gun games are coded to reject the official gun and will display an error message,"PERIPHERAL AT PORT A IS NOT SUPPORTED IN NORTH AMERICA", if you attempted to use one.
I recently acquired two official light guns with "US-MODEL" stickers attached. I tested them on the NTSC-U version of House of the Dead 2. Sure enough the games and light guns work without issues. The Dreamcast used for testing is a completely stock NTSC-U V0, there is no region or bios mod installed.
The source of the guns originated in San Francisco, HQ location of Sega of America. I suspect these were used at SoA for testing before they were canceled. I'm excited to have these, because I remember being really disappointed back in the day when it was announced that they would not be releasing the official light guns in North America. I had to settle for a crappy 3rd party gun.
Below is a video I recorded of the testing. Not a fan of youtube, so archive.org.