1) must be pump-action
2) cannot hold more than 20 paintballs at a time
3) must be powered by 12-gram CO2 powerlets
4) magazine tube must be parallel to the barrel for the "Rock and Cock" effect.
True stock class- horizontal feed tube, powered by 12 grams, no ball gates, check valves, auto triggers.
Modified Stock Class- Allows auto-triggers, and either a constant air tank OR a hopper (not both)
Super Stock- Allows constant air and a hopper, as well as check valves, auto triggers, and other modifications.
Open class- Pneumatic assist guns, anything goes, so long as you have to pump it to shoot.
Most SC games require the marker to be powered by a single 12g, and a horizontal feed tube that hold 20 paintballs or less. Angled feeds, spring feeds and hoppers are a no go. Auto trigger is a gray area because its negated by the horizontal feed tube. If you have it dont use it kind of thing. Lever changers, turrent changeers, and 6 packs are not S/C tho lever changers are usually allowed.
More info on S/C and how its changed over the years from the guys that play it.