[jmornoinf]

/Users/jasonmorneault/Desktop/Sig MCX/MCX Issue/IMG_4325.jpg

I had a similar issue before. Sounds like the bolt buffer in your lower is not interfacing with your new bolt carrier. You are going the have to replace the hard rubber "puck" in the lower in order for it to work properly.

 

[jmornoinf]

Rutherford,

I had the same exact problem. You need to call Sig and have them send you the new BCG buffer pad for your lower. The problem you describe happens when you pair a low serial number lower with a newer upper... Sig never told anyone they changed the design of the first Gen BCG and lower buffer. I have many MCX parts and pieces and now have seen three different BCG designs.

If I could figure out how to attach pictures I could show pics of what I am talking about.

 

[jmornoinf]

Does you bolt lock on the carrier like the picture? If so, you more than likely have an incompatible buffer pad. The rifle can fire in this state, but will not lock on a magizine when empty.