air weapons were excluded from the firwears act interpretation until a judge decided to redefine the lethal bit to prevent criminals possessing them and stun guns. There is a bit in the Act that goes on about noxious substances that was originally designed to stop people using mustard gas shells and lethal emans lethal to any creature so even if it is capable of killing a mouse it is covered and hence licenced in Scotland where air soft guns arent.
we had a gas gun at work that was designed to fire onbects such as ball bearings down to particles of dust using compressed air on one side of the chamber and a vacuum the other to replicate the effect of the impact of space debris with satellites etc. the velocity got up to 24000mph and even a speck of dust leave a big crater in a solar panel at that speed. Because it was an enclosed system it wasnt a firearm.
We had another verson for testing ceramic body armour and geophysical equipment that used a 10 bore shotgun mounted in a frame to fire into the ground to create shock waves. again exempt but needed by FAC to buy the ammunition. Ironically .22 rifles can also be exempt ( miniature rifle ranges like fairgrounds) and any firearm or weapon used in theatrical or film productions likewise. One of my brother's neighbours has a firearms cert because he restores vintage military vehicles and they arent always deactivated, not sure if he is covered by a museums licence
The term weapon is normally misused as well, the army have weapons, target shooters have firearms. I doubt if the crossbow was of military origin or issue but why let the facts stand in the way of the story