Go for a kalashnikov you can get one for the same price or cheaper than the MP 15-22, they are great for protection, you can buy bulk surplus amoo, and its always gonna work.
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=164260916
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=164271831
P.S. the 5.45 is close to a .223 and the 7.62 is on the far right in above pic