I carry an M&P Shield in 40 S&W. I chose 40 because its a nice hot medium weighted round. A pleasant side effect of owning and carrying a forty is the ammo availability. I am finding plenty of 40S&W ammunition at places that 9mm and .45acp are sold out. So that is great for me. All the PD depts in our area carry 40's and most Police in the US are being issued 40's. So availability in a SHTF senario should prove to be fruitful. Though not the cheapest round to plink with but certainly not the priciest.