In my experience, concealability ratings don't come up nearly as often as the GM saying "Okay, who's carrying anything bigger than a pistol? No one? Alright, no one hassles you." When the GM wants to disarm the team via weapon scanners and detailed security, that's what happens -- whether it's a machine pistol, a heavy, or a light, doesn't matter once real scrutiny is involved.
*shrug*
Mileage varies, as always.