I have a tweaked 1911 that I shoot regularly at matches and the past two weeks I have experienced the same failure; the firing pin stop-plate drops causing the firing pin to remain extruded and the weapon will not close into battery (probably a good thing with the extruded firing pin or I would have an eight shot machine gun). I had to complete each match with a different pistol and that really threw off my groove. I replaced the firing pin spring, thinking it might be a little weak, allowing the firing pin to travel too far forward but that did not help. I just replaced the stop-plate but the new one fits exactly the same as the old one and if I depress the firing pin beyond the depth of the plate, gravity will allow the stop plate to drop. I may be having a head-space issue with the primer strike letting the firing pin travel too far forward or not, it does not seem to be a bad fit and I have swapped firing pins but that did not help either. I am wondering if I should stake the base of the slide to arrest movement or if I should bite the bullet and build another slide (I really like this one)?
It is tuned, probably with a heavier hammer spring to improve lock time (I think it does nothing for more reliable ignition.) Did you get a heavier firing pin return spring to go with it? Chances are that the firing pin is getting driven way too far forward and the return spring pressure is not enough to get the pin returned before the stop plate drops. Have you checked the firing pin length (especially the hammer end) against specs? If it is too short, the problem is obvious. IIRC there is nothing holding the stop plate in place except the firing pin, so either you are right about a (huge) headspace problem, or the pin is not returning fast enough to return before the slide unlocks, or the pin is too short. Have you looked at the fired primers? Might have pin holes from the firing pin overtravelling from a weak return spring. If you upset the metal at the bottom of the slide to help hold the stop plate up, the slide will go into battery and you will have an 8 shot full auto. The slipped stop plate prevents that. A VERY good thing ---
I think I'll try another firing pin and spring tomorrow, head-space checks good with go and no-go. No sign of pin holes in primers and I have never had a miss-fire except when the plate drops. It might be that the striking surface of the firing pin has worn down enough to let it slip easily. Maybe one of those crisp new titanium firing pins will do the trick.
New firing pin, spring and backing plate seems to have fixed the problem; I'll know for sure after tonight's match
Good luck Sea I think that should fix it for sure, thats usually their only problem ever out of 1911's and an easy fix.