could be a rough / tight chamber or maybe gas leak at the front sight base
FTE can be related to gas port size but a stock AR 10 should be GTG
doesn't sound like a magazine related problem,
another possiblity would be the PO clipped the extractor spring to change the brass landing point. does it consistantly drop brass in the same spot?
one of the gurus on the AR list could prolly point you at the right location to test... http://www.ar15.com/ar15-l/
HTH
dZ
FTE can be related to gas port size but a stock AR 10 should be GTG
doesn't sound like a magazine related problem,
another possiblity would be the PO clipped the extractor spring to change the brass landing point. does it consistantly drop brass in the same spot?
one of the gurus on the AR list could prolly point you at the right location to test... http://www.ar15.com/ar15-l/
HTH
dZ