I have a new M&P 9mm.  The sear housing block is the newer style however it has the smaller 1/16th sear plunger and spring.  From what I've read I can't use one of the older style housing blocks that have the 1/8th sear plunger and spring because they wont fit in the newer style frame.  I've got the FSS kit installed with smaller plunger right now but I'm worried about sear flutter and other reliability problems.  Is my only option to send the housing block to someone and have them drill out a new hole?  Or are newer sear housing blocks with the larger hole available from S&W or another supplier?

Here's my sear housing block.

You will have to backorder through brownells or midway

You may also need a new ejector.

Ok.  My pistol has no safety or mag safety disconnect.  The only newer style sear block at brownells has a mag disconnect.  Is there a way to remove that part so it'll work?

I'm in the same situation and I plan to drill out the housing block to fit the larger spring and plunger.  It may be a week or two before I get around to doing it but I'll let you know how it turns out and I could modify yours if you're interested.