"Covert operations"? That would probably be something akin to a Welrod pistol like the OSS used in WWII...
Or at least some sort of small caliber silenced pistol that would be very quiet and effective at close range. Perhaps a Sig Mesquito .22LR with a modern silencer would do?
Far easier to find one and still legal to own with the right paperwork and tax stamp. But by definition, "operations" implies that you have some organized mission with specific parameters, which would logically dictate the choice of weapon(s) required.
Plus, it would also depend on what your support is, and if you plan on even needing to have a way out or not, etc.
If you are thinking of a "backpack gun", you might want to consider an AR-7 survival rifle.
Shoots a quiet .22LR, and the barrel stores into the stock in a waterproof, floating package. Plus, it doesn't really stand out as a threat to most people like an AR slung over the shoulder would. Of course, in a true SHTF situation, there might be times when it plays to your advantage to appear as a threat- especially to those predators looking for their next victim. Sort of the "yea; though I walk through the valley of the shadow of death, I shall fear no evil, for I shall arm myself enough to be the baddest mf'r in the valley!" approach.
But that all depends on the circumstances, and the destination, and how you plan to get there. And the key word here IS "Plan" because that is what will give you the best chance for you avoid the most fights and win the ones you cannot avoid. As has been mentioned above, though, I think what you seem to be thinking of is more a question of "What is the best mobile self-defense weapon for extricating myself from a dangerous situation?" than "What should I use for covert operations?" Not trying to be a smart-ass about it; but those are not one and the same in requirements.
