Again, it's technically a shotgun which Skirmishers were never meant to use. It might be buggy and some abilities may not function at all but if you really want you can edit the following file: Program Files\Steam\steamapps\common\XCOM 2\XCom2-WarOfTheChosen\XComGame\Config\DefaultClassData.ini and add AllowedWeapons=(SlotType=eInvSlot_PrimaryWeapon, WeaponType="shotgun")
AllowedWeapons=(SlotType=eInvSlot_PrimaryWeapon, WeaponType="rifle") lines to the classes that you want to use the weapons.

Posting this here because it gets asked a lot and I keep losing the location of the file you have to edit so I wanted a place to put it. If you don't feel confident in editing .inis you could probably easily install ADVENT cosmetics and make your rangers look like them.