Seeing the cannon almost destroyed a mountain I thought it was from a super advanced civilization, with mixed elements of modern technology and magic (because of his remark of sensing mana when the gun fired)
But why did it have to be a Yamato?
Why do you even want to send something like that back? Why don't you keep it?
Then again, I realized, keeping alien technology for research seems to be the start of a lot of horror movies. So can't blame them if they want to be careful.