As per the screenshot below, the cannon drill doesn't actually care if an unbored cannon part is facing the right way, and will drill the cannon part through its side. One would expect the cannon drill to check if the cannon part's end is facing towards the drill bit and refuse to drill if it isn't.

I think I've actually encountered this before - mostly thought no-one would really notice. However, this could probably be solved with a direction check or something like that. I'll take care of it later

Fixed in e2be52c