Lets Diagnose the problem:
I know you stated you lubricated the parts well but I will include it in the below information also!
I presume you are speaking of an AR-15? First of all good lubrication is a must! With a good lubricant notice I did not say grease! Breakfree, G-96 or other good quality lube! The Bolt Carrier and Bolt should look a little wet at all times! Secondly, try some other full power ammunition! If that is not it try a New Magazine (of quality since there is a lot of junk on the market!) Has this just started happening? Is it happening with one certain magazine or all magazines? When it happens to start *Mark the Magazine/s and see if it happens again with this same magazine/s. If it does you may have diagnosed your problem! "The Magazine/s". if the New one works! The magazine is the number one cause of this problem followed by improper lubrication, followed by ammunition. If the rifle has always done this we can fix that also but try proper lubrication, ammunition and then a new Mag first and we will go to the other possible fix later if that is the needed! One other thing have you changed the stock on it to a different full stock?
Since the rifle is ejecting and loading correctly it should not be a short stroke problem other than the possibility of under powered ammunition.
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Last edited by Sniper03; 07-14-2013 at 12:27 AM.