Slangers and halve slangers

In one Dutch document from 1665, the 12pdr guns are referred to as "slangers" and the 6pdrs are "halve slangers". I had thought that they might be slings, but I am not sure that is correct. Iron guns are often described as "gotelingen" and large bronze guns as cartouwen (24pdr) and halve cartouwen (18pdr). Often, they are just called "pieces" (stukken).