There is no easy way to buy craft weapons on the black market - they're strictly regulated. And getting a custom-made weapon onto an airframe is certainly harder than just bolting it to the hull. At least if you want it to work. Long story short, now we can, and nobody can do anything about it. These weapons are called 'light' but remember, that even 50mm cannons are huge compared to personal arms and we better stock up a lot of material if you want to manufacture them.