Launching a projectile at a direction relative to the "barrel"?

At the moment, in my script a projectile is launched in a way that uses X, Y and Z. It would work well if the cannon was stationary, but it isn't. It's supposed to be on a ship. How do I make it so that the projectile is launched relative to the "barrel"?

Here's the script I have at the moment:

https://pastebin.com/6yiTeYPW

On line 12 you see what method I use.