There is always the Jack Bauer route; tactical "attache case". It's actually not such a crazy idea if you plan and train around it. Many good-sized ones can hold quite a few primary and secondary mags as well as other stuff, plus it would be very quick and easy to don in a pinch. Combined with a subgun-sized SBR that could fit inside it, you'd have a discreet and useful setup.......for you know.......when you're at the library.........and Chechen terrorists take over the place and you have to shoot you're way out or something. 
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