Adam, Jef
I simply recall reading the line that "for the purposes of the act any (my emphasis) projectile weapon which causes (not is capable of causing) death or significant injury shall, for the purposes of the act, (this was in the middle of a part of one of the firearms acts and my brain was hurting) be classified as a firearm regardless of the method of projection of the projectile.

The entire thing is riddled with technical bull excrement. For example my neighbour and I have adjoining gardens with a clear view between them. If I put a target in his garden and he joins me in a spot of plinking I am breaking the law but he is not. I am "allowing or causing projectiles to leave the boundaries on MY property", but he is firing into his property which, since he has my permission as the property owner of the property he is shooting from, is not illegal.

Malakiblunt,
ther was a medievil cross bow with a barrel for shooting bullets (though most bullet bows did not have barrels)
It was called a slurbow and there was a spring powered gun which fired a quarrel or bullet but I can't remember what it was called.

Regards
Jim D