The Prototypical 
Guide To 
Paintball Guns 
SEMI-AUTO GUNS
Z-1 / Z-2

 From the collection of Thad "Weltman" Drum
Manufacturer Montneel Designs
Action Blow Back Semi-Auto
Configuration Bore and Valve In Line
Valve Style Tippmann Based
Caliber 68
Velocity Adjustment Gas Block Type
Feed Type Direct Feed
Barrel Brass Lined Barrel 
Set Screw Retension
Gas Source Constant Air Adapter
Features: Options:

Protobable - As you probably have gathered, I got the features from a Montneel Designs pamphlet.  It was probably written before Nitrogen became really popular.  Even after Nitrogen hit the scene, Factory reps called running on Siphon C02 Tanks the "Poor Mans Nitrogen".
                This gun was popular to copy.  I can think of at least 4 guns that were virtual copies (there were probably more).