1.Pull the buffer out the receiver extension and while shining a light down the tube, confirm that the extender butt stock screw is not protruding into the back of the receiver extension void.

2. Cleaning and CLP lubing an AR rifle are a bit different that other rifles.  Take look at either your manual or check out the cleaning section her at the site to get you up to speed on the weapon.
http://www.ar15.com/forums/forum.html?b=3&f=7

2. Some ammo's can be problems with new rifles until they have a chance to break in, especially if they are not cleaned correctly and the upper bearing surface lubed correctly with CLP.  To add, even mags come into play since some of the cheap clone knock off's are just less than desirable.  When buying mags, you want to buy only USGI spec'd mags.

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