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?

According to what I've read if you are feeding 1 spent casing and 1 live round it's an extractor problem.

But then again that is just what I've read.

If you are sure that the carrier key screws are tight,
www.ar15.com/forums/topic.html?b=3&f=66&t=249259

Sounds like the chamber is fouled, rough and needs to be correctly lubed, and mantained to allow the chamber to break in (read self polish threw live fire).