A paint ball gun includes a barrel, a handle with a trigger connected thereto and a tubular portion which is located beside the barrel. The tubular portion includes a chamber in which a strike pin and a valve are located. A rod movably extends through the valve and has a first end and a second end. The strike pin hits the first end of the rod by pulling the trigger and an activation member is connected to the second end of the rod. The second end of the activation member movably extends into a tubular member engaged with the chamber and biased by a spring. A stop member is movably engaged with the tubular member and in contact with the second end of the activation member. By adjusting the compressing of the spring by moving the stop member, the pressure that shoots the paint balls can be adjusted.