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Originally Posted by Scorch
Kinetic energy= 1/2 mass X velocity squared
Momentum = mass X velocity
You may notice in both equations, mass figures prominently. The mass of the hammer is the primary cause of induced vibration. In older cartridge rifles, the mass of the hammer is a carryover from percussion rifles, heavy leaf springs, and thick primer cups. John Browning demonstrated that the heavier hammers were not necessary, his 1878 design that became the Winchester 1885 High Wall had a much lighter hammer, and dominated target shooting in the last decade of the 1800s, beating out the Sharps, Remingtons, Ballards, Wessons, and Marlin rifles.
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The hammer doesn't store any energy so it can't be a source of energy - it simply transfers the energy stored in the spring. The equation above is one means of calculating that energy, but it doesn't imply the hammer's a source of energy. If we knew the spring constant, we could use a hammer-independent equation to calculate energy. And since energy's supplied by the spring, it's constant, no matter the mass of the hammer - decrease hammer mass by 20%, and hammer speed increases by 12%. Energy remains the same, but power increases 12%, while muzzle-jarring momentum and lock time decrease by 10%.
At any rate, my experience with revolvers has convinced me there are manifold benefits to a lighter hammer. There is a practical limit, though, and not knowing enough about AR15 design, I understand those hammers may already be closer to that limit. The basic idea (and physics) is the same, though.