[John Sabato]

Okay, based on your last post, this is a sideplate interface with the upper receiver issue. Apparently the L-shaped lever is not performing it's job correctly. This may be the result of some previous owner working on the trigger pull by polishing one of the contact surfaces of the "L". If too much material has been polished off, then the lever can't supply a consistent push to the sear bar.

From your photo of the inside of the sideplate, it appears that the upper part of the "L" has not been messed with. As an experiment, I would suggest that you cut a small piece of durable tape, (like the shiney aluminum tape that is used to seal duct work ---no not cloth duct tape) and tape it to the top of the lower L surface so that it will increase the lever movement by the amount of the thickness of the tape. This will not be durable, but will provide more information on how to correct your problem.

With that piece of tape securely mounted, reassemble the gun and see if you have consistant firing pin drop. For the amount of testing, just use a fired cased with the old primer still installed... a snap cap may be overkill. Dry firing is not recommended as a frequent activity, but dropping the firing pin a few dozen times a year really shouldn't hurt anything.

If the addition of the tape on the lower part of the L proves to successfully allow dropping the firing pin, then at least we will know the source of your problem, and I can suggest a few possible long term remedies...