[Surestick]

Originally Posted by xkpxfatassxx

How many lbs of pressure or force does it take to shoot a .68 paintball approximately 300fps. I was assuming 50lbs because the arrow bow requires a 50lb pull sting, but I can see that it's most likely different. So does anyone know?

Your question isn't clearly defined enough.
F=ma or force equals mass times acceleration.
Assuming you know the mass of the paintball and the speed you want it to reach to know the force needed to accelerate it to a given speed you need to know the acceleration (otherwise any force starting at the drag on a paintball at 300 fps and up will eventually accelerate the paintball to 300fps).
Without specifying barrel length to determine acceleration your question has no real answer.

The pull strength of the airbow isn't directly related to the force on the ball, the distance the string of the bow moves is just as important (50lbs over 1 inch is significantly less energy than 50lbs over a couple of feet). Contrast the 50lb pull strength of the airbow with the 800+psi input of a marker that operates on unregulated CO2 (say a lower end Spyder) and then contrast that with an Airstar Nova that fires the ball using only 90 psi.

What you really want is the curve of the force the bow string applies as it goes through its travel. Graph that over the distance it travels & the area under the graph gives you the amount of energy needed to accelerate the paintball to 300fps plus the energy needed to overcome friction & other losses in the airbow itself.