Sam Vimes will seldom be alongside the Assassins' Guild on any question, but they are in general agreement about inflicting violent deterrence on anyone using a spring-gonne. Sir Samuel says weapons are "for having. For being seen. For warning. They're so you don't have to kill people." A spring-gonne, on the other hand, is a crossbow without the cross, a slim tube with a bit of handle, easily concealed under a coat, even in a pocket. It's an assassination weapon.(1)
The bow has been replaced by a spring working in compression (or conceivably in extension, like many spear-fishing weapons). The bolt is fired from a tube perhaps less than a foot (30cm) long, giving it limited range and accuracy. It is the equivalent of a Derringer at a card table. The device is also known as a "one-shot", because considering the difficulty of cocking again and the range at which it's useful, if the first shot isn't effective there won't be another.
The Guild of Armourers have developed a host of ingenious missile launchers based on energy stored in metal, wood or horn springs but the idea of storing the potential energy in compressed air doesn't seem to have developed.
(1)The Assassins have nothing against powerful concealed weapons. They are,however, strongly opposed to other people having them.