M&P 40 Consistant misfire on first round

kingjr5872 said:

Ok, I had the same problem with my M&P .357 sig and my M&P Shield .40. I actually have three of these weapons and have broken all of them down to the last spring and pin. Here is what I have found.
First don't let the slide slam forward if you are not chambering a round. The slide will bounce and bang across a key component in the receiver that serves two functions. Luckily if you have done this there is an easy fix, provided you are comfortable stripping your weapon down to the trigger and sear block. Some new weapons are afflicted with a similar misfire failure which can be fixed with this procedure.
There are three parts that you need to become familiar with. You can go to the Brownell's sight for a schematic. Two parts are shown in the receiver, the trigger bar assembly and the sear housing block assembly. The last part is the safety stop plunger which is shown in the slide underneath the rear sight. The safety stop plunger is not identified in the schematic but it is depicted, along with its cap and spring. All of these parts will need to be removed.
The trigger bar assembly has a candy cane shaped piece of extrusion. This piece relaxes or flattens over extended use or may not be set properly from the factory. This piece of extrusion is what allows the trigger bar to interact with the sear.
Also on the trigger bar is a cam shaped piece next to the extrusion piece. This cam interacts with the safety stop plunger in the slide. Both the cam and the plunger wear with use. The cam also has some resistance moving across the chamfer cut on the plunger. The trigger bar is cold stamped from sheet metal causing the cam to have ragged edges (look at it under a magnifying lens) All of these problems can cause misfires. The bottom of the extrusion also determines the height of the cam relative to the sear housing block. This then determines to total interference between the cam and the safety stop plunger.
The fix. First (using automotive feeler gauges) measure the gap on the trigger bar extrusion (the inside of the candy cane). It should be no less than .015 inch and no greater than .018 inch. I recommend .016 inch as this fixed my first misfire issue. This modification will allow the sear to lift high enough during trigger pull.
Next you need to smooth the edges of the trigger bar cam and the safety stop plunger by polishing. Be careful not to remove any material from the top of the cam and the flat bottom of the safety stop plunger. You can round the plunger but leave material on the very bottom. These two distances must be maintained to ensure that the safety stop plunger is positioned high enough during trigger pull to allow the firing pin to release.
There are a lot of videos out there from Apex and some other independent people explaining how to break your weapon down to these components. Be sure and get the Apex Armorers block, it's the best armorers' tool you will ever buy for the money.
About me - I am a senior manufacturing technician and I have a unquenchable thirst for knowing how things work. I will be starting a blog soon on how to make your own tools and instruments for working on your weapon(s). i.e. don't use a round punch to remove your rear slight. Whoever first recommended this is plain stupid. Round punches and rectangular components don't match. Grind or file a 3/8 inch punch down so that it has a flat surface and use it instead.

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