Kousenjuu, Kōsenjū or Kosenjyu?

The Japanese name for the Nintendo light-gun series is 光線銃 SP, which means "light ray gun SP".

光線 --> light ray
銃 --> gun

But what is the correct romanization for these kanji? That should not be too difficult.

光線 is written "kōsen" or "kousen" if we write "ou" instead of the long "ō"

銃 is written "jū" or "juu" if we write "uu" instead of the long "ū"

Together this forms "Kousenjuu".

Nintendo however writes it as "Kosenjyu" on the guns.

Kosenjyu? Wait a minute. Is that correct?

Is this an error? Or an alternative romanization?

Who has more information about this?

For now I will stick with "Kousenjuu".