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Well actually there is a big difference in how Pumps work vs how Semi's work which makes for a massive difference in accuracy. In a Semi, the ball is dropped into the breech from the hopper and it sits there, the Bolt is in the Open position, held by a spring (your general Woodsball markers) and the ball is resting against a ball detent to prevent it from rolling down the barrel. When the trigger is pulled, the sear releases the hammer which is forced forward by a spring, it is linked with the bolt in front. It hits the valve, opening it to let the air or gas explode out, and the bolt hits the paintball, slamming into it and pushing it past the detent and into the barrel followed shortly by the expanding gas that propels it down the barrel and the shot is made.This causes a complete change in inertia on the paintball and the gel / liquid inside by first being hit violently by the bolt (that's why tournament grade paint tends to breaks in a Woodsball marker) and following that the gas hits it. Many different forces enact upon the ball, making it slightly more unstable.Now in a Nelson design pump, the bolt is held in the closed position. Pumping it moves the bolt and hammer backwards, allowing a ball to fall into the breach. Then the bolt returns, leaving the hammer behind, sealing the gas expansion chamber. The bolt is no in the closed position, gently resting against the paintball that has been pushed into the barrel. Many older Pumps didn't even have a detent and the balls could just roll out. When the trigger is pulled, the sear releases the bolt, which in turn opens the valve (doesn't slam into it) and the gas expands, rushing through the mechanism and shoots out the resting paintball. No bolt hitting it, just the air / gas. It begins its journey down the barrel less violently and this results in a constant acceleration on the paintball being the the only force it experiences, instead of a spring powered bolt hitting it first followed by the air acceleration of a Semi.This is what makes a Pump more accurate. I've personally played with all types of markers, Open-Bolt Blowback, (Tippmanns), Open-Bolt Flex valves (Tippmann X7 Phenom and PE Etha), Poppet Valves (Etek / Ego 10), Spoolies (Dye) and a Nelson Type, Closed Bolt (Hammer 7, Empire Sniper, Armson, AirowGun)... I have tried numerous barrels, Bigshot, J&J Ceramic, Tech-T iFit, Shaft 3/4, Knightstick Carbon Fibre, etc... And of all the markers I can honestly say there is a major difference in accuracy between Pumps and everything else. It's just a matter of physics, less forces enacting upon a gel filled sphere results in less inertial play on the fluid inside. Bolt + Air, or just Air...
How to play woodsball
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