So, as far as I understand it, the bullet's on-screen vertical offset is not
computed, but use the value defined in the .offset file of each bullet images
(meaning that it does not take into account the weapon used to fire the bullet,
since the same offset will be used for all weapons using that type of bullet).
I don't know how those offsets where initially computed...
I will try to change those offset values to reflect the new Tux model, but we
definitely need a better fix, after 0.15.