Jerry, GREAT observation! As many times as I've manually compressed the firing pin spring by flexing the breechblock and middle link and I completely let that fact slip my mind.
My thought would be that compression of the firing pin spring under recoil is necessary because the recoil spring power plus the momentum of the mass of the train wouldn't be enough to compress the spring reliably - maybe not at all - through engagement of the sear near the end of the cycle when recoil spring pressure would be nearing it's lowest point.
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