According to its source, this shaggy man is firing a .510 caliber (13mm) single-shot pistol.

Pmnht0A - Imgur

Its round? A .500 Nitro Express — ammo designed for taking down elephants and other huge game in Africa, AKA a low-key weekend for Teddy Roosevelt.

More on the big daddy round from Wikipedia:

The .500 Nitro Express is a .510 caliber (13 mm) big bore hunting cartridge. It was designed for use in single shot and double rifles. The cartridge is available in two lengths: a 3.00 in (76 mm) and a 3.25 in (83 mm) version. The cartridge was designed for use against large dangerous game species in Africa and India.