Stock class paintball follows the same concept as stock car racing: everyone uses the same level of equipment so the competition is based on the skill of the player. It has grown as a movement by players who see the technology of paintball becoming a substitute for skill. Stock class players enter the field with a limited amount of air and paint so every shot must count. Players win by relying on marksmanship and movement instead of "accuracy by volume".

The following guidelines have generally been accepted as the standards for a "stock class" paintball gun:

Feed/Operating System

• The maker is powered by a single 12 gram CO2 cartridge
• The marker must be manually actuated (i.e. pumped) to load a ball and cock the markers action. Semiautomatic or double-action markers are not allow (with the exception of the Crossman 3357 Spotmarker)
• Paintballs are gravity fed from a tube parallel to the barrel
• The marker must be tilted to load each paintball (No direct feeds)
• The feed tube cannot hold more than 20 paintballs
• The feed tube cannot be modified to impede the balls from rolling off the bolt with the exception of the thickness of the main body and the tube above it
• The magazine tube or feeding block's outer circumference must touch the outer circumference of the barrel and must be parallel to the longitudinal axis of the barrel. It must be flat with no ramps to help balls roll to the bolt. (This is to prevent any design that provides enough space to stack paintballs and be ready to load without tilting the paintmarker.)

Power System

• "Quick changers" for the CO2 cartridge are not allowed. The knob holding the CO2 in must be unscrewed and the 12 gram dropped out through the threads. (With the exception of the Nelspot 007 and Crossman 3357 Spotmarker.)
• The knob must be turned at least one and one half full revolutions to remove it. Check valves are not allowed. For safety reasons, removing the CO2 must inactivate the marker
• Phantom stock class, Rat-a-tac, and other "bucket-type" quick changers are permitted

Barrel

• Holes, rifling, or muzzle brakes are not allowed. The barrel may be any length but must be smooth bored and solid
• There can be one barrel addition but must not exceed eight inches from the tip of the (original marker's) barrel and may not be drilled, rifled, etc.

Miscellaneous

• Autotriggers are not allowed
• Any type of stocks, grips, and sights are allowed with the exception of Battlegrips for the Nelspot which have a drop out hole for the twelve gram cartridge

Courtesy of http://www.warpig.com - www.warpig.com

are you sure u can't have porting in the barrels? 

Styro Folme wrote: are you sure u can't have porting in the barrels?

No, I'm not.