Guys, recoil springs and hammer springs are rated at .005" from stack (where the spring is collapsed down so coils touch coils), so this means that a 17# reciol spring is exerting 17# of thrust at the point is is almost fully collapsed. To be real accurate we would have to include friction of slide to frame, slide over magazine and ammunition, AND amount of magazine tension thrust on the slide...but in general it will take about 17# of weight hanging off the back of the slide of a glock to move the slide back to the rear...more for hammer guns cause we need to consider the amount of hammer spring to overcome and the amount less the fucrum of the hammer piviot pin...but once again, in general this is what it will take.