while at the range today with my newest rifle ( my older essential arms lower with a new dpms a2 dcm upper) I started barrel break in and after about 25 rounds (with gi 20 round mags that have previously worked perfectly in the rifle) the last round in the magazine started popping out of the mag and the gun locking open with the round laying on top of the follower. any ideas on what the problem might be? i plan on taking a new magazine with me to the range and trying again- could this be just a break in problem that will fix itself? thanks

thanks for the response Dano. I took apart and cleaned the mag in question as well as taking two other mags with me and the rifle is now 100%