I recently built a new ar with a midstate firearms 300 blackout upper, and an anderson arms bolt.  I was having an issue with each round extracting but being dropped on the next round.  This would cause the bolt to close on the empty case above a loaded round.  they would both be pushed and wedged into the chamber and jam the gun.  I found the below post, which is now archived, but it didnt have a solution.  I contacted the original poster and he stated he had a low power extractor spring that caused the issue.  I added an o ring and the problem has been solved.  The rifle is now running 100%.  I hope this helps somebody else.

http://www.ar15.com/forums/t_3_66/592821_brand_new_build_double_feeding_not_extracting__and_jamming.html#

Great tip...
Thanks!