User blog comment:JoeRamirez/Call of Duty Modern Warfare 3 Weapons Wishlist./@comment-216.40.83.245-20110202213611/@comment-28584497-20110204010121

...FMJ is when most (with the exception of the part concealed by the neck of the casing) of a bullet is covered in copper so the round doesn't change shape as much or shatter on impact as easily.

Total Metal Jacket is similar, but covers the ENTIRE bullet.