I bought a new Smith & Wesson MP 10 in .308, and have had the same failure to eject Problem. This is my first 30 cal AR So this is all a learning curve fo me.
I shot 20 rounds of Remington Core Lok 150 gr without any flaws. The 3rd magazine started ok, then I started having failure to eject, failure to load and failure to lock after the last round. I talked to the dealer and he told me that he has always experienced the need for a break in period on his .308s.
I went home, cleaned, oiled the BCG and the counter bore/ bore. (Wet) Back to the range, 20 rounds of Winchester 180 gr shot a nice group. Then same problems on the next 10 rounds. I called S&W, and with out question they sent me a shipping label. I decided to try one more time at the range. Another 40 rounds still had similar issues. So I have fired a little more than 120 rounds through my AR10 with about a 40% failure to eject, feed and lock after last round. I am going to send it to S&W and let them determine what the problem is.
Side note: I have used 4 different MagPul a one HexMag Magazines.
