Pull all the way back on the charging handle, and make sure that the front face of the bolt does not retract back past the back edge of the ejection port window.
The face of the bolt should stop 1/8" to 1/4" in front of the back edge of the ejection port window.

Next, shotgun the upper open, and hand cycle the B/C in the upper receiver with your finger in the back of the carrier (charging handle in the upper locked home) and work the B/C back and forth to see if there is any binding.

Lastly, pull the bolt off the carrier, and with just the carrier and key, dry fit the carrier to the upper.  I bring this up, since I am seeing scrapping on the side of the carrrier key, and it almost like the gas tube is not correctly indexed to the carrier key.

If the gas tube needs to be tweaked, then tweak it over the center of the barrel to align the back end of it with the carrier key.

When the buffer bumper compresses on impact to the back of the tube void wall, this will put the face of the bolt behind the ejection port window, with the spent cases pivoting off the bolt face at rear buffer stall  tagging the back of the ejection port window edge.  When it gets bad enough, the spent cases are going to be deflected back into the action to cause a jam instead.  So, the receiver extension needs to be threaded into the lower receiver another thread (or the buffer is too short instead).  It looks like you have a fixed stock, so try another buffer to weed the buffer in play out as the problem and being produced too short, and it the buffer is not the problem, then its the receiver extension instead (void milled too deep by 1/8" to 1/4").

The receiver extension is threaded 16 pitch so for every wind its threaded into the receiver, the distance is .166".  So the face of the stop collar surface against the back of the receiver needs to be faced .166", which will allow the receiver extension to be threaded into the receiver one more wind, and regarding the front bottom lip, then just need to be faced back the needed amount so the lip just kisses the center post of the buffer retainer with the tube torqued correctly.




As for the gouge scratches on the top/back of the key, that is steel against steel marks, so chances are they came from a blown primer piece (anvil or cup) that got between the key and bottom of U slot in the charging handle.  With luck, the blown piece of primer was blown out off the action, but might want to look under the trigger for it hiding there.




P.S. Did you check the charging handle, hence it locked home, and how far you can retract back the B/C past the back edge of the ejection window before the back of the key hits the end of U slot in the charging handle?

Not a fan of wasting parts/money, so unless you can get the factory defected part replaced for free, easier to just bring the rifle back into spec by mod'g a part  instead.

What is the length of your buffer, and what is the depth of the receiver extension from the front end of tube to the back wall of the inner void?  
If these are on the money, then may be the thickness of the threaded section for the tube on the lower receiver that is too thick instead.