I got my new piston upper the other day and just had a chance to install it on my LMT lower today.  It's a very tight fit on the lower, and when I pull the charging handle back the bolt doesn't release forward.  With a slight tap of the barrel to the ground it releases but I can't figure out why it won't release forward naturally.  Any help would be appreciated.

is there an empty mag in the gun?  Will the bolt go forward when you push the bolt catch (ping pong paddle)?

Negative.  No empty mag and doesn't go forward when I push the bolt catch.  When I pull it back it feels like it's rubbing on something and then it just stays back.

1. Can you try the upper on another lower?  That might help you determine which part is causing the problem and help you narrow down you trouble shooting.  I'm not sure by your post if the lower is new also or not.

2.  Is everything lubed up really wet?  The bolt carrier has pads on the bottom to keep it from tilting.  These need to be lubed up with grease on my LMT piston.  I made sure the receiver extension had a light coating of CLP in it as well.

3.  Are there any signs of wear in the upper receiver?  Take the upper off and turn it upside down.  Move the bolt carrier back and forth and see if it's catching on something.

It's probably just the MRP platform.  I reported this when I got my piston upper a few weeks ago. My LMT piston and DI platform seems to have a hard time with my S&W and RRA lower(I just gave up after a while). The piston upper will fit on any of my 3 LMT lowers without problems.

I check to make sure if it's just specific to the MRP platform. I tried a regular LMT carbine and it fit in my S&W and RRA.

I then check the MRP upper to see if it will fit in a Colt and Bushmaster. Yes on the Colt and No on the Bushmaster.



edit: sorry didn't read the full post. The only problem I got with the MRP platform is not able to fit with "certain" lowers. Can't produce the same error with my LMT lowers that work.

That seems very unusual.  My piston MRP upper went right on my LMT lower.  It was snug at 1st, but after a few times the pins have worked in fine and it comes apart easily.  And it functioned right off the bat without a hiccup.  

The only thing I can think of is if you installed the H2 buffer, maybe the little pin that holds the buffer and spring in is somehow hanging up on the BCG.  Maybe take the whole buffer and spring back out and re-install and see if that helps.  Maybe something got kinked when you put the H2 buffer in.