A side AU project showing AU USAF, US Army, and USN operators of the Lockheed Martin C-27J Spartan and associated variants.
The USAF operates the aircraft in the tactical airlift role with regular, reserve, and state air guard/air militia units both in the US and around the world (replacing older aircraft like the original C-27A, C-123 and Ahrens AR 404/C-23A); in addition AFSOC operates a dedicated SF/CSAR variant known as the MC-27J Praetorian and TAC operate a dedicated electronic warfare and attack variant designated as the EC-27J Compass Pacer (massive heads up to The_Sprinklez/etccommand for the inspiration and some drawing elements for this particular variant, much appreciated). A training version with defensive aids suite/countermeasures removed is used for the training of C-130J pilots and aircrew.
The US Army operates the aircraft for organic tactical airlift and utility roles in support of forward deployed forces such as in Europe and the Middle East. US Army C-27Js are also used by various state guards/militias for domestic support roles (such as in Alaska or in support of the USACE), by USASOC for special forces support roles, and by the US Army Airborne School at Fort Benning for paratrooper training.
Finally, the US Navy operates a number of specially modified HC-27J Ocean Sentry aircraft for maritime SAR roles such as rescue co-ordination, airdrop, and patrol as well as in a secondary MPA role. Basic C-27J models are used by both regular and US Navy Auxiliary Reserve Fleet Logistics Support Squadrons for regional airlift and utility roles transporting personnel and equipment, as well as by Fleet Marine Logistics Support Squadrons for USMC troopship roles.
[edit: minor corrections, recolouring on MC-27J wings, fuel tanks on MC-27J and HC-27Js, and maritime search radar on HC-27Js.