The gas pistols of the category NBB (Non Blow Back) have no moving slide. However, the next BB is automatically reloaded after each shot. The advantage of NBB guns is that they consume less gas and they are quiter due to the absence of moving parts.
Models which are also equipped with a silencer are popular because they are considerably more silent comparing to gas blow back pistols.
NBB pistols are somewhat easier to maintain than pistols with blow back because less wear and tear occurs. Most of these arms are made of plastic. Unless noted to the contrary in the product details those weapons are generally semi-automatic.
All guns are supplied without gas which must be purchased separately. In the category Gas & CO2 it is described which gas should be used for which gun.