Because that ammo is everywhere! You totally missed the point chief. In all realness 9mm, 45 cal., .223, 7.62x39, .308, and 12 gauge are your best bets.
Sure, people make their own black powder. But, how many actually go out in the field and dig up the components and make it successfully? I'd rather just stockpile ammo, which I've got a great start at, thankyewverymuch!
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I child-proofed my house, but they still keep getting in!
I recomend getting a copy of "Improvised Munitions Handbook" Army TM 31-210. it has several recipes for black powder and other easily made from scratch propellents and where to find the raw materials to make them, how to reload primers, how to use nails for bullets, home made guns and lots of explosive recipes and booby trap designs. I have one of the original un censored versions of the little black book and it is a treasure trove of info.
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peace through superior firepower