You can be pretty sure that a submariner can give you a competent explanation of what "Rig for Dive" means
Well maybe. I get your point and in general agree. Being in the military can mean alot of things and not necessarily "Firearms Expert".
But it depends on the individual as much as MOS. I know SEAL's that aren't gun people (not saying their not Good at what they do, just not all are gun enthusiast).
As for the bubble head above. Maybe he is a gun guy, shoots competitively, attends the Nationals for Service Rifle, and is on the PRP program (Personal Reliability i.e. Reaction Force that guards nukes).
My point is, it depends on the individual more than job title. Military, Law Enforcement or Convenience Store owner.