If you've got the money for special containers and bribes, you're not even going through the checkpoint, you're going around it. To actually go through a checkpoint, with a gun unnoticed, the largest thing you can get through there are pistols completely unnoticed, maybe an AR in a false bottomed MDS sheathed suitcase, maybe. But inspectors aren't the slouches we'd like to imagine they are.
But like I just said, the point is to prevent people from automatically noticing something, you can be inconspicuous with things. You can look like you're just carrying a beach umbrella, when it's actually packed with a rifle, or look like you're just wearing a poofy skirt, when under it is a sawed off LMG. Not getting into a swaray with it, but walking down the street across the way from the hidden Renraku building disguised as a condemned tenement, no one's going to bat an eye.