A lot of players consider the Combat Shotgun to be one of the best and easiest guns to use in Rage 2, so you will definitely want to get your hands on this weapon as soon as possible. Luckily it's not that rare of a weapon, and you can get it pretty early on.
In order to get the Combat Shotgun, you will need to go to Gunbarrel and find Marshall. He will give you a mission called "Blackout" where you must enter a sewer to find and Ark, and the Combat Shotgun will be located in this Sewer next to the Ark. Enjoy the weapon!