I wouldn't call Carters rare. There are quite a few floating around.
Anyway the Box Gun depending on what you get will take them about 4 months to finish.
The bores are HUGE on the Box Gun but you can buy the optional barrel kit they have.
Blowfish was a pump team from way back that have been out of the game for a few years, I'm not sure if they are back to playing but they don't have their store anymore (I think).
Your best bet to getting a Box Gun is buying on here or from Carter Machine directly.