Just to add ;

If you put mags in a protector case and reload to one of the mags in the protector cases, it creates a bug where the mag stays in the protector case, while being in the gun also. The ammo will drain from the mag in the protector case when firing.

However, If you log out, and back in, you will get full use of both mags again .. This works for all mags, including stripper clips and is an old bug.