Okay, first things first. I have no way of knowing what's happening with
your M&P (as a M&P pistol armorer) without examining your particular pistol.
...
At this point I reassembled the firearm and dry fired it. Then when I locked the slide back to see if the striker safety plunger was engaged it was not.
That's odd. At first I was just going to post the normal striker safety plunger check armorers do, but then I thought I'd try to duplicate your unusual "test", according to what I thought you're describing in the above quote.
I pulled one of my M&P's from the safe and tried to duplicate what you wrote (in the quoted section) about dry-firing and checking the striker safety plunger condition. My striker returned rearward and the safety plunger returned to the normal position (popped out fully).
I couldn't duplicate what you described ...
until I removed the striker return spring from my striker assembly.
Then, installing the striker assembly sans the striker return spring, after dry-firing and retracting the slide, locking it back, I checked the safety plunger. I consistently found I had to pull back on the striker a bit (which was stuck forward more than normal) in order to reset the striker safety plunger (let it pop out). Does that sound like what's happening in your M&P's?
Take your M&P's and remove the slides, so you can look into the opening at the rear of the striker channel. Look immediately forward of the feet of the strikers, at what's exposed on the striker assemblies. Can you see a couple coils of the striker return spring in all the guns? If not, and the spring is missing in the affected guns, that's a problem.
If all the striker return springs are present, however, the cause is probably elsewhere. (As an armorer, I'd still remove and disassemble the striker assemblies, to double check for broken, damaged or defective striker return springs.)
The next thing I'd check would be the striker safety plungers and their machined channels. This means removing the rear sights, so it's a job for someone with a sight pusher and some experience.
The current safety plunger
spring is a 1-piece unit incorporating the
spring and the
spring plate, unlike the previous design where the spring and spring plate were separate parts (and could make for some occasionally challenging reassembly). It also reduces the chance for an improperly positioned
top coil of the spring (such as being bent or folded over), since it's now attached at the bottom of the spring plate.
Might still be a problem with how the
bottom of the spring was inserted into the plunger, though. Dunno, but some Glock owners have sometimes caused themselves problems with improperly inserted/set safety plunger springs.
Might also be something gummed up inside the plunger spring and/or plunger channel, hindering freedom of movement of the plunger.
Also, as someone else mentioned, I'd check for any burred edges of the plunger hole, to make sure the plunger moved freely as intended. If this is the case, I'd send the guns back and let the repair techs do their thing under warranty (and carefully document your concern and what's you've experienced in a letter included with EACH boxed gun). Don't assume.
Letting the repair techs examine the guns would also let them eyeball the strikers and plungers (and replace them if anything looks remiss).
Dunno, just some thoughts about where I'd start.
Now, as far as normally checking the functioning of the safety plunger? A S&W M&P pistol armorer verifies the striker safety in a M&P pistol is functioning properly using these steps:
Remove the slide from the frame of an empty pistol, because the checks are done with the slide removed from the frame.
*Pull the striker to the full rear position and slowly let it forward. (This sets the proper striker/plunger relationship.)
*Push the striker forward. It should NOT move past the striker safety plunger and protrude from the breech face. If does, it's recommended the armorer replaces the striker (and probably the striker safety, but S&W likes to be very cautious about such things, so the striker is listed as being replaced). Re-check to make sure the problem has been corrected.
*Next, push down on the striker safety plunger and hold it down. Push the striker forward. The striker should move forward past the safety plunger and protrude from the breech face.
Now, if you don't initially pull the striker to the full rear position (and then let it slowly forward until it stops), it's sometimes caused some owners some concern when they found could push the striker forward. What's actually happened is that they hadn't known the striker had to be positioned in a certain relationship to the safety plunger to even begin the check.
FWIW, I'd be a little surprised if you were missing a couple of striker return springs, but weirder things have happened. The striker assemblies (like the trigger bar assemblies) are assembled in-house from vendor supplied parts. If the parts dept person(s) misses a step (striker return spring) ... an "oops" slips out the door.
But hey, I stopped being surprised when I encountered my 2nd or 3rd .45 magazine (3rd gen mags) which had the mag springs installed upside down and backwards.
Not something I would've expected to happen ... but it did. Sigh ...
Let us know what you find and decide.
