Grip and rip the mag. The rounds in the mag will shift enough that the round which is hung up will be freed from the assembly. Then lock the bolt back, clear the jamed and unfired round. Let the bolt go forward and cycle the bolt until the stuck case comes out.
Take a good look at the stuck case and make sure you don't have a worn chamber by looking for any gas burns on the case.
If you can't get the stuck case out with the bolt, lock the bolt back and very carefully, paying close attention to not damage the muzzle, use a cleaning rod from the muzzle end to kock the case free from the chamber.
Examine your bolt extractor and make sure it's clean and ensure that the extractor tooth isn't broken off.
If you are worried your going to damage your mag, taken another mag and load some rounds in it. Put another round across the gap between the feedlips and push down...it should load into the mag. Congrats, you just saw that the rounds will move as you rip the mag and you just learned how to properly load the mags instead of trying to slide rounds in front to back.(we've all been there
).
Wes