the glassgun is a blowbun enchanted to fire small & sharp peaces of glass when it is blown with NO amunition in it!
it can be used 12 times a day,for future reference.
not sure what type of spells would be used in making it though.
The Crystal Dart would be a fair starting point, although it relies on Muto rather than Creo to actually "create" the dart.
As a concept this isn't bad. I once imagined a crossbow that created it's own arrows and fired them (A creo/Rego effect).
The crystal dart is only designed the way it is so that virtually every Te-Fo combo had at least one attack spell. I think that one dates back to 3rd edition.