He must have been a FFDO otherwise, how would he have gotten the ammunition past security personal at his home base. I guess security could have missed it.
FFDO has nothing to do with it. More likely he did not go through security, but went through a KCM portal.
More likely is he used that bag for a trip to the range with his personal gun, and failed to empty it completely. That part is understandable and forgivable. However, his actions on the airplane are not -- they were idiotic and dangerous. He even had a second chance when the FA found them in the bin, but again chose idiocy.