C-Products Defense and D&H mags are specifically designed for the x39. I have a truck ton of ASC mags that run as well, but most of them are still in their packages. I primarily run D&H mags.
I would have a 5.7x28 rifle, but only if I could build one... I don’t want to just slap on an upper. I’d like to build up a DI version and run it off FN Five-Seven mags. I got a chance to shoot EyeDoc’s AR-57 and was impressed. It definitely fits the part of a high capacity yet low profile rifle system.
An AR-57 short barrel upper, would be awesome on a pistol lower receiver that is chopped/designed to use AK mags. Super low profile. Maybe even get silly, and add a FAB defense folding pistol grip. That would be one hell of a truck gun. Screw putting optics on it. I would go super low profile again, and find some HK MP7 or CZ Scorpion sights.
I agree with Tom. I would pack absolutely everything I had… To start. Realistically though, if you don’t have a 12ga, a 5.56 rifle and a 9mm pistol, you will not be prepared when your surplus runs out. I love all my 308, but I know that 223 ammo will be on the cost ratio of 10:1 to 308 ammo. 12 gauge, 9mm, 45acp, .223 and 762x39 are the default cartridges of the apocalypse.