Rifle is short stroking.
But lets back it up,

Pull all the way back on the charging handle, and make sure that the face of the bolt stops about 1/8" to 1/4" in front of the back edge of the ejection port window.  Also, during the last inch'ish of the charging handle pull all the way back, make sure that the tension on the charging handle is not increasing in tension. If the tension is increasing, then chances are the recoil spring is too long, and the spring is coil binding out (recoil spring length should be around 10.5" long for  telescoping butt stock tube) .

Next, shotgun the upper open, pull the charging handle and B/C, pull the bolt off the carrier, and with just the carrier with key and the charging handle installed, work the carrier in the upper receive to make sure that the carrier is not binding in the upper receiver, and the gas tube is correctly indexed with the key.

Now with the upper still open, lock a empty mag in the mag well, and make sure that the empty mag follower is raising the bolt catch all the way up.  If the catch will not rise correctly, drop the mag, and move the catch to hand to make sure that its not binding in the receiver slot (should just be the spring tension pulling the catch back down, but the catch should move freely in the receiver slot.  


On the B/C, spray some CLP around the base of the key, hold the bolt in, and use an air compressor with air gun to pressurize the front of the key and make sure that there are no leaks between the bottom of the key and the top of the carrier.


If everything is good to this point,
Use a chamber brush with CLP to scrub the chamber by hand, then use dry patches to get the chamber and bore dry.
Note, should be using CLP not only to clean the rifle (with the exception of the bore, since something like Sweets will clean the copper fouling out quicker), but also to lube the upper receiver bearing areas as well.  Also, before the rifle is short, it needs to be cleaning with CLP (including scrubbing the chamber as well).  The new rifle comes with assembly/storage grease to prevent parts from rusting during long term storage before the rifle is sold, and this Grease needs to be removed before the rifle is put into service.

On the cleaned B/C, apply CLP on the heaver side both inside and out, give the B/C a quick flick, insert the B/C into the upper, lock the upper home, then empty cycle the rifle a few times to migrate the CLP to the upper receiver bearing areas.

On the mags, pull them apart, and give them a Good CLP cleaning as well.

As for the buffer, you should using a standard 2.9oz carbine buffer for the telescoping butt stock system.  A H buffer should not be needed for the standard Ball type ammo you are firing.

Now back at the round, load a single round in a mag, insert the mag in the well, charge and round while leaving the empty mag in the rifle, and fire for effect.  The correct effect should be the bolt locking back on the bolt catch (catch in front of the mag, and not only on the bottom of the carrier).

Get me this far with test, and if you still have problems, then post back and need a few photo's of the spent cases so I can see the side of the rims, and the area on the spent case rim where the extractor is pulling the spent cases out of the chamber.