i'd argue that the 22LR is the ultimate SHTF gun. you can fit probably 5k rounds in ACU cargo pockets and with a m&p15/22 and a half dozen 25 round mags, you would be a formidable foe. granted you ain't not match for a 223 range wise but i wouldn't want to be within 200 yards of the other end of my 15/22.
