Unusual M&P CORE returning to battery problem

[groovyash]

So I have a new CORE 9 that is exhibiting an unusual malfunction, and one I've not experienced in any of my other M&Ps. When cycling the slide slowly with the trigger depressed completely the slide will hang up about 1/8 of an inch out of battery. I can either nudge it the rest of the way forward or if I slightly let pressure off the trigger at which point it will pop the rest of the way closed. Without the trigger depressed it cycles fine. Live fire I have only experienced it three times out of a relatively low 200ish round count but it's the same issue. I've cleaned up the trigger bar, striker block, striker contact surfaces, and sear thinking there may have been a burr somewhere with no success. I also tried swapping out the striker block and plunger spring with an Apex USB which resulted in a really horrible, two part, mushy reset so I swapped it back. I'm really at a loss on this one. I'm not a stranger to the platform and armoring them but can't seem to get this to run right. Any thoughts would be appreciated.

[Clint]

You can isolate the sear & striker by activating the yellow takedown lever.

My guess is it's got to be the striker block having trouble getting up the ramp on the trigger bar.

[groovyash]

Thanks Clint, I should have thought of that!

So with the sear deactivated the gun cycles fine by hand returning to battery completely every time. I was kind of thinking it was the striker block as well but this would seem to indicate the opposite. I'm more puzzled now that before.

Edited to add: With the recoil spring and barrel removed I can cycle by hand and feel the change in pressure in the last 1 or 2mm of forward movement. The first stage of pressure is pretty obviously the striker cocking and normal so I'm not sure what is engaging in the final distance.