recently bought a Bushmaster M-4 (AR-15) and test fired it today. Initially, firing went fine, then after about 20 rds, I started getting double feeds consisting of the spent case and the next round in the mag. My guess on this is the extractor might be a bit rough and is holding onto the round, or the ejector is bad. I tested the ejector (pushed in on it and it has no slack, feels nice and firm), so I can only think it is the extractor. I fired Black Hills ammo (68 grain moly coated hollow points)through it. Any ideas?