Made it to the range again today for the second trip with my new S&W M&P15.  Had some trouble again with the trigger not resetting.  After the shot, the case would eject and a new round would feed into the chamber but when you went to pull the trigger nothing would happen.   Sometimes after a second or two you could here a clunk and feel the trigger reset and then you could pull the trigger and it would fire fine. Other times, I could tap on the side of the lower receiver and it would clunk and you could feel it reset and you then could pull the trigger and it would fire.   Still others, I had to pull the charging handle back, ejecting the chambered round, feed a new round and then it would fire.   This doesn't appear to be a FT Feed or FT Eject.  It also happened with the two kinds of ammo (AE .223 55gr and PMC X-Tac 5.56 62gr)  I had and in all 4 2rd Pmags.

This happened multiple times on each of 4 20rnd mags I shot today.   Sometimes it would be fine for several shots and then happen on each shot.  Pretty frustrating. There's only 120 rnds fired on this factory M&P15.  It seems like something is just keeping the trigger from resetting fully some of the time.  I am a newbie so I am wondering if there is an easy fix to this or should I just send it back to S&W?

Thanks!

Check the springs on the hammer and trigger. It sounds like you are getting a failure of the hammer to reset properly.  Find a diagram of how the springs should look when fitted on the hammer and trigger and check yours to make sure they are placed properly.

I had the same problem. everything looked fine I had a smith look at it I can't remember the whole explanation I just opted to change the whole trigger out.

much happier now.

Sounds like you may be missing the disconnector spring. I had the same thing happen when I first finished my AR build  and the damn thing went full auto...then single shot.

Not had a problem with it going full auto...  yet!  

It either works as it should or there's nothing when you try and pull the trigger.

So I was just playing with my lower and looking it over. The springs appear to be installed correctly  and the disconnector spring does appear to be there as well.

If I pull the trigger and hold it back and then push the hammer down again and then release the trigger I get the same symptom of hammer is back but pulling the trigger does nothing. I did find out though that if I push the trigger ever so slightly forward, it will reset and then it works as it should.  What I don't know or can't tell is why this just doesn't happen on it's own when I release the trigger.  What is not allowing the trigger to complete is forward movement thereby resetting the trigger.  Also pushing down ever so slightly on the hammer also seems to free it up and allow the trigger to complete the reset.

After watching this video it appears that trigger is having a hard time getting into the little slot at the bottom of the hammer by itself.  could there just be some rough edges there?  Or does it have something to with the springs?

do you have a set of reduced power springs? I had the same problem with my set of lighter springs, swapped out the trigger spring for a regular and now works fine...... just a thought.

EDIT: mine worked fine for about 1k round count then all of the sudden went to shit.  same problem, would work fine for a few then go to shit.  so my conclusion was that the reduced power spring kept well reducing with it being used(like every other spring and compression cycles).  note: I did check all the same things that evey one here is suggesting and like you everything checked out.

I had the same problem with my new S&W. The interface between trigger and disconnector were so roughly (badly) machined that the disconnector would not release the trigger every time without pushing the trigger forward. This happened most often when I released trigger slowly. Polishing the disconnector and trigger hooks (please see the image below) fixed it for me. Hope this helps you too!

http://farm8.staticflickr.com/7353/9425369358_4d5c87e150_o.jpg

Glad I found this thread. I was at the range today shooting my first build and was having this same problem.

Disassemble the Lower Receiver, and clean it squeaky clean.  Dry fire/cycle at least 200 times (cushioning the hammer so it doesn't impact the bolt catch or Lower Receiver).  Clean throughly AGAIN, and lube generously with CLP.

This is a new gun.  It needs at least 500 full power round (M193 or M855) fired through it to start the break in process.  Don't expect the gun to perform anywhere near your expectations until then.

The Upper Receiver Assembly parts need to break in as well.

This is insane.  I've never done this on Any gun I've built for myself or anyone else.  There is no such thing as AR break in.  USGI triggers will settle in a little, but there is NO functional problems on a properly built AR from round 1 fired in the gun.

All newly assembled mechanical devices require a break-in period before they become reliable (guns, cars, toasters).  Frictional parts need to mate to each other, and machined surfaces need the edges and machining debris knocked off, and the finish smoothed out.  I'd venture that if everything you every built up from brand new parts assembled perfectly, without fitting, or break-in, then the tolerances were too high, and sometime not in long in the future, the whatever it is you assembled is going to be sloppy or loose.

this.