Backwards, brother. DPMS puts a K at the end of the serial number on stripped lowers, only. If it has a K at the end, it started life as a stripped lower, and not a full factory rifle. :thumbsup: There's either a gas system issue going on here, or a recoil system issue (weights of buffers, or spring rate/length). OP, need more details. LOTS of details. As in, list every part of the rifle. Pics help.