APEX DCAEK installation and measured before and after pulls - it depends...
I'm not sure this was clear, so let me be sure. The APEX DCAEK didn't do in my M&P. Others have seen their trigger pull go from 6.5 to about 5 - 5.5 lbs with this kit. As C hawk Glock mentions in his reply, my gun has a light trigger to start with for what ever reason, so I didn't see as much change that others have.
The trigger on the M&P 9mm could be better. According to APEX's video, stock triggers on the M&P have a trigger pull from 6 - 6.5 lbs. My completely stock M&P 9mmf has a trigger pull right at 5.5 lbs. The lighter pull can be attributed to 'burnishing' of trigger parts by just shooting/dry firing it.
For me, 5.5 lbs is quite acceptable, hardly worth spending $90 for a DCAEK kit unless it really does something special. My largest objection to the M&P trigger is the deplorable reset. Reset characteristics will mean nothing to some, some may not even be aware of what it is. So, mostly out of curiosity, I spent my $90 for a DCAEK and installed it this morning. I wanted to do this in a progression and measure the impact of each step.
I started with just the hard sear and trigger spring; I suspect those are gonna contribute the most to the trigger action. Here's the result of that:
The measured and averaged 'after' showed a reduced trigger pull somewhere between 0.25 and 0.5 lbs. That's about a 7% decrease in pull weight. However, while I had the gun disassembled, I polished the trigger bar ramp that contacts the sear - it needed it too! That may have influenced the 7% decrease as much as anything. So the trigger went from 5.5 lbs to 5-1/8 lbs, again, about 7%.
The reset is still deplorable. The hard sear and trigger spring did little, if anything, to improve the tactile reset. So the thing that needed the most help, didn't get any help. At least not so far.
To me, on my gun, the trigger return does seems to have more reset force, i.e. a stronger trigger spring, but the reset 'click' remains essentially the same which makes sense as the reset 'click', like a Glock reset, comes from the lateral spring acton of the trigger bar, not the trigger spring. It also makes sense that the sear, sear plunger, and sear plunger spring cannot help the tactile feel because the actual reset comes when the trigger bar slides off of the side of the sear.
This reset mechanics explains why/how the RAM improves tactile reset where the DCAEK cannot. The simple operation of the RAM is to put more lateral pressure on the trigger bar so when it slides off the sear, it hits harder. APEX explains the impact of this in their RAM installation video.
However, the APEX FPB is reshaped and polished; maybe that will reduce some sliding friction, so I installed it next. Measurements revealed no measureable reduced trigger pull weight. Oh, and I polished the trigger bar where it contacts the FPB.
Trigger reset? Still deplorable. I cannot sense any change in the tactile feel of the trigger, and I now have everything but the plunger and plunger spring installed. But as I've already discussed, the sear, etc. cannot affect reset impact. Tactile reset remains unchanged and is still a non-event.
I realize not everyone gets the same results, and even APEX admitted to me that since my gun was already at about 5.5 pounds from shooting, not much would be gained. They were exactly right. My trigger measures about 7% lighter and no noticable improvement in tactile reset.
Unfortunately, my M&P has the thumb safety version of the sear housing even though it does not have the thumb safety. The significance of that is, that I could not install the RAM kit. The RAM kit is designed for the guns with a non-thumb safety type housing.
Hmmm, which means I could install a thumb safety if I wanted to.