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Beiruty wrote:Shouldn't firing pins be sprng-loaded so the pin is recesed unless hit by the hammer?

when dirty enough they can stick. The M&P in question had seen a few thousand rounds and had the FP channel oiled after every range trip. This is one reason people should never oil the FP channel on a firearm.

I saw an M&P slam fire when the firing pin channel was dirty. What happened was the owner had a habit of oiling the firing pin channel and after enough time oil and residue built up causing the firing pin the hang a little forward despite the firing pin block. I suspect something similar happened here, so my belief is that either the firearm is defective or the firearm was improperly maintained.