Ok, I'm trying to figure out exactly how recoil operated guns actually work and have hit a point of confusion. I assumed that the momentum of the bullet (integral of force with respect to time) was the same as the bolt, that the bolt then recoiled into a spring. I assumed that the kinetic energy of the bolt carrier was transferred into the spring over the length of the action. Calculating all this out to find the spring constant, it looks like it would take two hundred pounds of force to work the action. So I must be missing something somewhere. I've heard terms like "mass delayed blowback" and wonder what other elements might be involved. I'm familiar with the roller-delayed blowback system used in my HK, but it's the simpler ones that are baffling me. Doesn't look that complicated. Here's what I was trying to work with: Bullet mass * velocity = Bullet momentum = Bolt momentum Bolt momentum/bolt mass = bolt velocity (Bolt velocity)^2 * mass / 2 = bolt energy Bolt energy = spring energy = spring constant / 2 * (x2^2-x1^2) Distance between x1 and x2 is the length of the action. Calculating for a 45 ACP 230gr at 500m/s 4 kg*m/s momentum Divided by 0.25kg bolt mass (about half a pound) = 16m/s m*v^2 / 2 = 32J of kinetic energy KE=SE (spring energy) = K / 2 * (0.1m^2 - 0.05m^2) <-- (change in compression of the spring) Spring constant K turned out to be 8533 N/m Meaning holding it at the 0.1m (10cm, ~4in) point would take 853N, or nearly 200lb. Anyone know where I went wrong? I guess the greater question is WHY??? Well I wanna build one.