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Some obscure spell might 'reduce' his BS to 1 for a turn, allowing him to fire it. If someone can ever kill something with a a Staff of the Lost Sun on an oldblood then I will personally eat a whole obsidian weapon!
With modifiers you can get a 0 BS or worse. You need a 7 to hit (7-0=7), therefore a 6 followed by a 4 witout any other modifiers. not to great
I really don't think shooting is an option, just see the "7 to hit" and an old White Dwarf Pirate Zombie army all with BS 0 and guns, was kind of pointless but they still shot.