Black Box Projectile Launchers with Payloads

When you try to make a black box projectile that fires a payload (nuke, flak, etc.), the module designer tells you that the payload is longer than the barrel of the weapon. There is no way, as far as I know, to fix this, because black-box guns don't have a barrel, and because this is a "red" error, the module becomes unusable as a result.

Thanks,
Rabbitflyer