How long has that gun been sitting or last cleaning?
You might have to disassemble the inertia arm/link and rear stock light oil lube (i.e. check for goo). Clean it (rcvr too), loosely reassemble, and then cycle the action ... tighten it up slowly/evenly. Also make sure the link is not contacting the rcvr-stock plastic spacer (seen that a time or two on M1 SGs).
I had one that acted up, and this brought mine back into action 100% ... just a thought.
Last edited by SPIN2010; 01-05-2013 at 05:02..