I bought a used Smith and Wesson Model M&P15T Rifle [AR-15 style] which I was told had about 500 rounds through it. It looks only lightly used... Today I took it to the range for my first time and had some trouble which I need some help with. Of the first 30 round magazine, all fired and ejected without a problem. After the first 3 or 4 fired from the second magazine it failed to fire and the cartridge would not eject. When I opened the weapon, the cartridge fell out. The projectile was scratched and primer untouched. After 3 successive failures, I packed up and went home. I didn't find any obvious problem when I cleaned the gun. What might have gone wrong or what did I do wrong?