Gunsmithing Conundrum? Any Ideas?

[FS1]

Long story as short as can be
Background: I have an MR556 rifle (love it) and recently I bought a MR556 Upper (for clarity I will call it a CR upper). Also Bought a HK forged Brownells finished 416 lower. Installed LMT internals and everything from the buffer stop pin back OEM HK. HK grip also.

Put the CR lower on the Brownells lower to function check. Cocked charging handle moves freely no trouble. un-cocked charging handle locks up tight and won't budge. So first I assume I made a mistake. I double and triple check everything nothing obvious. even the hammer height is exactly 1" on both.

So I put the CR upper on my HK lower functions perfect. I start thinking maybe LMT not compatible with HK. To confirm I put the MR upper on the Brownells lower expecting the same malfunction. But no, it functions perfectly on the Brownells lower. WTF

Has anyone ever seen something similar or have any advice to offer. I finally put them down and stepped back to reflect for a while.

 

[ShooterwithNoName]

I've heard of a similar problem with HK uppers. Where the HK BCG has a firing pin safety you have to use a full profile hammer. Is the hammer your using in the lower notched? It will do what your describing if you have a notched hammer in the lower.

Also... check your buffer retaining pin? I'm just thinking over anything that could cause it. Any pics of your lower and trigger?

 

[FS1]

I removed both BCG's and disassembled and noticed no problems. I reassembled and the rifle functions now.

I do not know what cleared the malfunction. This fact will drive me crazy. Now I just wait and see if it returns.

 

[goatboy]

I removed both BCG's and disassembled and noticed no problems. I reassembled and the rifle functions now.

I do not know what cleared the malfunction. This fact will drive me crazy. Now I just wait and see if it returns.

If the BCG on the MR556 is like that on a 416, check the bolt. The carrier is spring loaded to reset and may be hanging up.