While back I posted on a problem. a AR10 in .338 jamming when feeding from the right side of a magazine
http://www.ar15.com/archive/topic.html?b=3&f=66&t=692257

After trying various things, handed the barrel to someone with the right tools.  He checked the headspace(correct), and gave the chamber a good polishing.  I finally got it back and reassembled it yesterday.

After some cycling tests with dummies worked, fired it today.  Only about twelve rounds, but kept topping-off the mag so it would be feeding from the right for the first half-dozen, and zero problems.  Looks like that was all it needed.

Right side feed presentation malfunctions (FTFeed) are often:

* bad feed lip geometry of the magazine

* Sharp ejector that catches the side of the case rim  (This will manifest as a failure to go into battery)

* Improper alignment of the magazine with the feed ramps in one or a combination of any of the axes, due to tolerance stack