All good points, e.g. I seem to remember reading that a 747 can maintain pressure with up to 6 windows being open (not that you'd want to be next to any of them

All good points, e.g. I seem to remember reading that a 747 can maintain pressure with up to 6 windows being open (not that you'd want to be next to any of them!), but I think bullets still wants fairly dense, and that tends to make them more X-ray opaque, right? Hmmm, if we're just talking about single shot guns, you don't need to align cartridges in the characteristic rows of magazines, or the cylinder of a revolver, making them easier for the TSA to miss. Of course, you could just load a magazine once on the plane.