At the range yesterday I tried out my AR-15. Ive had it quite awhile but havent gotten around to shooting it till now. When I tried to insert the magazine it would not lock into place. P-Mag, no go with bolt closed, bolt back or downloaded magazine. Surplus mag, locked in place with bolt open but had hard time with bolt closed. Downloading the mag did nothing to help. I tried numerous mags with the same the same results. Any ideas other than send it back to factory?