I have several different calibers AR (22lr, 9mm, 5.45x39, 5.56, 7.62x39, 6.8 spc, 7.62 NATO) if I have to pick one all around fun/SD would be the 9mm because the price & availability of ammo. My kids prefer the 9mm & i enjoy the 7.62 NATO. My next project is 458 SOCOM.
"Real liberty is never found in despotism or in the extremes of democracy"- Alexander Hamilton