What was a World War 2 Expert Carbine Sharpshooter?
The US Army classifies firearms qualifications in three classes. Expert- top scores, Sharpshooter- high ability but not expert, Marksman- the lowest ability but still qualified. You may mean the M-1 Carbine issued to officers and soldiers not on front line combat duty. It was a short, light weapon firing basically a 30 caliber pistol round...intended for short range and without much knock down power. An Expert Carbine Sharpshooter was someone who had scored a minimum set score on the firing range with a carbine at a certain distance. "Sharpshooters" qualified with different weapons and some with several weapons. Each soldier who passed the qualification test for firearms qualification was issued a special badge that was worn under the ribbons on the LEFT HAND pocket. The Marksman badge was a simple Maltese cross. The badge for the Sharpshooter was a Maltese cross with a target superimposed over the center of the cross. The Expert badge was a smaller version of the Sharpshooter cross with a wreath around it. During WW2, each of these 3 types of qualification badges had small loops under them for a "hangar" that was a small rectangle plaque engraved with the weapon that the soldier qualified with. It was possible to be qualified in more than 1 weapon skill and so an additional plaque was hung under the the previous one.
The weapons that were the badges were issued for included the following (this list is not all conclusive). RIFLE
BAYONET = Not sure how you qualified for this 1.
PISTOL-D = Cavalry troop firing dismounted
CARBINE-D = Cavalry troop firing dismounted
75 RECOIL RIFLE
There was one for anti-aircraft gunner. Another type of Qualification badge that was issued was for Drivers. This too had hangars that displayed the type of equipment that he qualified for.
Then another was Army Aviation Mechanic Qualification Badge. I think the answer you are seeking is that an Army Expert Shooter in the Carbine classification was a soldier who had met the highest grade of qualifications for the Carbine (most likely the M-1) on a firing range. This qualification did NOT have any relavance to his combat experience.