...

Ammo used?

federal xm-193, lot 30, I have used about 100 round of this lot and haven't had prior problems so I would be hesitant to say it was ammo related

check the extractor and spring,and is the empty especialy hard to poke out if so could be headspace too tight or the barrel nut too tight was the barrel indexed at the factory or some other?My advice try changing the extracor spring first to a heavy one if you have a carbine if its a full size rifle try the same if it still happens change extractor,if it still happens send it to the factory and let them work on it,in my area theres a guy who was an armorer for over 20 years in the marines,he builds match M1s M1As and ARs hes the only one besides bushmaster Id trust the rifle to.If its pulling the empty out but the carrier is stopping halfway in its rearward travel you could be experiencing gas loss wich could mean your bolt gas rings not staggered,missing,damaged or your carrier key is slightly loose.

It is pulling them out and stopping halfway, I would doubt it is the extractor because there are only 300 rds through the rifle, the carrier key is properly staked and bolt gass rings are properly spaced

If, as I suspect, the work required that the gas tube be removed and reinstalled then possibly the stress of putting it in and out has caused it to crack where the pin goes through it in the front site.  Check for signs of gas leaking there.
Mario_FAMOInc

Scrub the carrier key area with bore solvent I didnt know mine was the same way until I cleaned it ,it was only loose 1mm or less it was very slight,take an allen wrench to the screws and make sure if not it could be the gas tube in any event your getting gas loss somewhere.