AP/API bullets to reload for the .50 BMG. Powder and primers. Food. I'm good on the gun situation. Might add some more odd parts and firing pins for the firearms I have. Oh yeah, and a 55 gallon drum of CLP.
A Remington M40-XS, a top of the line AR pistol, a top of the line AR carbine and tons of ammo. Then put the pistol upper on the carbine lower for a SBR and sell the leftover components. At that point I think the feds would have bigger problems then me not having a tax stamp.
I would buy a 50 calibre bolt action rifle with a matching scope and a couple of hundred rounds of 50 Cal BMG AP ammo. If I had any money remaining, I would just spend it on ammo (.308, .223, 7.62x39, 12ga 00 buck, .44 magnum and .45ACP). Other than the 50 calibre, I would not complicate things by adding additional calibres to my collection.
Then again, I might just use that same money to quietly stock up on all other necessary survival supplies and try to not draw any attention to myself by buying firearms and ammunition since who would know what would soon happen if such an order were issued. I might just want to visit my cabin in the woods for a while.
A truckload of spare parts and magazines for both, some cosmoline (or heavy grease) and plastic wrap, and as much 5.45x39 as I could get before the money ran out.
And a shovel....and some magazines to pass the time before its time to dig 'em up.