 The Nelson design is "in-line" which means that the bolt, hammer, and valve follow the barrel and form a line as opposed to being stacked.It fires from a "closed bolt" position meaning that when it fires the bolt is secured into the breech end of the barrel with a paintball loaded.In comparison, the bolt in an "open bolt" marker is not secured until the marker fires, at which point the paintball is chambered, fired, and the bolt is then re-opened.