Have a local smith (familiar with S&W metal-framed pistols) clean out the firing pin channel.
Removal of the manual safety assembly and firing pin is needed for that task.
Someone unfamiliar with the process may easily reassemble the gun improperly, or even damage parts.
As an armorer who supported approx a thousand 3rd gen's over the years, I had to clean out accumulated fouling, debris and outright sludge from firing pin channels and firing pin springs.
The good news is that I also inspected early vintage 3rd gen's which had been in-service for many years, since they were new, and the firing pin channels were completely dry and free of any fouling. Just a very light dusting of carbon on some spots, but nothing which interfered with normal functioning.
The common cause of a nasty, fouled firing pin chamber is improper owner/user cleaning practices.
Letting excessive liquids (solvents, CLP's, oils) run into the firing pin channel can easily result in the trapped liquids attracting and accumulating fouling, debris (primer cup shaving and brass shavings from the case rim edges) and any other contaminants. Those things can eventually turn into a heavy sludge which will either interfere with the firing pin's freedom of movement, or lodge between the firing pin spring's coils, interfering with its compression ... or both. Light-strikes result. Pretty predictable.
We started traching our people to hod the slides pointing muzzle downward when brushing the breech faces, so gravity would help keep any solvents or CLP's (depending what we were using at any time) from running into the firing pin hole.
Naturally, it also helped not to excessively "drench" a brush in liquid and slather it on the gun. (You're not cleaning your mag wheels.)
Then, making sure excessively applied liquids weren't introduced into the other entry points for the firing pin channel was also helpful - meaning the rear of the firing pin; ... the sides of the manual safety assembly/levers; ... the bottom of the manual safety assembly, meaning the "cylinder" you see rotate when you move the safety levers; ... and the bottom of the slide where the steel and plastic plungers are located (firing pin safety lever and ejector depressor lever). All of those places are potential entry points to the firing pin channel.
I'm not a particular fan of aerosol cleaners, either. In my experience they can present a couple potential issues. First, if you use a spray product intended for automotive application, such as brake cleaner, it may cause a rapid temperature change that can create condensation ... and then that condensation can't escape the space in which it's trapped, so you can risk rust, which is never good thing for the firing pin, or the springs which tension the 2 plungers in the slide, next to the firing pin channel.
Secondly, while a forceful spray can force its way inside those small confined spaces, the resulting liquids - and the loosened fouling they may carry with them - may not easily find their way
back out.
A
lightly moistened patch (think a couple of drops), wrapped around the wide or narrow head of a
dry brush, can loosen, lift and clean away a surprising amount of fouling. Without saturating the surfaces of a gun like you were washing a car.
FWIW, some of these same considerations apply to cleaning the extractor hook. Don't introduce excessive liquids that can become trapped underneath and behind the extractor. A dry brush can clean away fouling behind the hook.
Also, when you're cleaning under the slide of a S&W 3rd gen 9/.40, notice the thin machined slot along the right side of the center pick-up rail? That opens into the extractor recess. (The .45's don't have that opening.)
Letting excessive liquids run inside can let them accumulate - with fouling, debris, contaminants, etc - and they can also start to thicken or solidify, which may introduce some extractor functioning issues.
Too much buildup behind the hook, at the front of the extractor (just behind the opening), and failures-to-extract may occur (because the extractor can't close enough to get a firm grtasp on the rim anymore).
Too much build at the rear (under the tail, where the extractor spring is also located), and failures-to-feed may occur. Debris may accumulate around and within the extractor spring, which can interfere with the normal compression of the spring, and the extractor tail movement, so the front of the extractor may no longer be able to open wide enough to accept a case rim sliding under it.
One thing I started doing over the years, to demonstrate to some of our people how they were using an excessive amount of liquids in "cleaning" their extractors, was to take them into the armorer's bench, where we had an air compressor.
I used the narrow nozzle (with them wearing safety glasses) to blast the air upward, directly into the narrow machined slot on the bottom of the slide (under the extractor recess, remember?) ... and let them be totally surprised by the mass of black liquid and sludge that suddenly came out from all around the top, bottom and rear of the extractor (looking at the extractor in the side of the slide). Making sure the compressor wasn't able to blow condensation into the confined space, using the compressor, was important. Moisture and small springs aren't a good combination.
Gunking up firing pins, extractors, plungers and all of their related springs, is unnecessary and undesirable, all things considered. Easy to prevent, too.
Just don't introduce liquids to those confined spaces as the result of inattentive or improper cleaning practices.
Yes, if the gun ever becomes contaminated, such as if dropped into standing or moving water, it really ought to be disassembled and cleaned out by a smith (or S&W pistol armorer, if you happen to know one). I've seen what can happen with a 3rd gen if someone submerges it (think sea water
) and decides to wait a year before reporting it. Rust never sleeps.
Just some thoughts.
I do a lot of engineering substantiation research on my day job and have to be mindful of productive/unproductive information. 
