A very similar question was posted on another AR15 site, and this was my response:
Worthwhile upgrades to a M&P15...
A properly staked M-16 BCG (the heavier M-16 BCG will help reduce felt recoil) Sprinco Blue Spring Kit (includes an action spring and an extractor spring for your bolt) H2 buffer
When you change to a M-16 BCG, you really should improve the spring and get a heavier buffer at the same time to avoid recoil problems, and that's why I recommend the spring kit and buffer. Expect to spend about $200 for all three items. This worked in *my* MOE.
Addendum to original text: There are several special coatings available, but in the end, they don't really enhance the functionality of the rifle or the efficiency of the action itself. A standard coating on a BCG will be sufficient (that's what's on your M&P). Like I said, the special coatings only make cleaning a bit easier. Is that worth the extra cash? Not in my opinion, but that's just *my* opinion.
I would also recommend upgrading your trigger. Sometimes, you can get away with just changing to lighter springs (JP Enterprises sells spring kits for $9). This worked in *my* MOE.
If you want more weight on the trigger than the 3.5 pounds afforded by the JP springs, get a Spikes Tactical "battle trigger" for $54. It consists of a complete fire control group and results in a 6# trigger, as opposed to the 10-12# trigger in your S&W. I have one of these trigger groups in my homemade AR because the JP springs were too light, and it feels good enough that I want to put the same trigger in my MOE. Since neither of my ARs are match-grade rifles, I see absolutely no point in buying foo-foo triggers that can cost hundreds of dollars.
Finally, get a set of KNS anti-walk/rotation pins (about $32). Some will say you don't need this item on most ARs, but rack the bolt, hold the rifle up so you're looking at the hammer pin on the lower receiver, and pull the trigger. The pin will rotate at least a 1/4 turn, and this will wear out your hammer pin hole (steel pin + aluminum lower = wobbled hole).
After you've done all that, the rifle will cycle smoother, recoil softer, and fire more reliably/smoothly.
As an aside, my MOE was working fine before I did all that stuff. Mostly, anything you do to an MOE is feel-good stuff. I feel really good about my MOE.
Addendum to original message: Someone mentioned an accuwedge, and I've never heard of that being needed in a S&W. My upper/lower fitment is very tight with no wiggle at all, but my homemade AR did have wiggle. Instead of using an accuwedge, I used a set of YHM quick-takedown pins, and it tightened up the fit quite a bit (but now all of it). It still wiggles a bit, but that's not going to hurt anything.
There's also a much cheaper way to address receiver fitment. Just put an o-ring on the upper receiver lug (the lug that the front takedown pin goes through).
The only other upgrade that will affect how well your rifle shoots is a floating hand-guard. That gives you several options regarding configuration, but thta's a discussion best left for another thread.
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