My "non-smithed" M&P will fire when the slide is back about 1/8".
As I mentioned, when using a 13# ISMI spring mine would not completely go into battery yet pulling the trigger would release the striker. That was enough to dimple the primer and fully close the slide giving the impression of a light primer strike. Thankfully I was using CCI primers or I might have had a K-boom like yours. That happened about 6 times during a big match and I was convinced it was light primer strikes until I got home and did some closer inspections.
What was your reload? Bullet, powder and COL?
3.6 V320, MG 147 JFP, Federal primer, mixed range brass.
Interesting that yours will fire with the slide back 1/8" also.
I have an XD and an XD-m that will "fire" with slide back about 1/8", but I would think that since the round is not up against anything in front of it at that point, wouldn't the stryker just put the round forward? I'm gonna make some powderless rounds and see if it will ignite the primer...