One does not need a sniper course to become a sniper The fact is the Army DID have snipers by name and trade in WW and while they filled a role that today would be called marksman that was what the term was in the Army in  Furthermore by his own discussion in the Caparzo death scene thats where Id be if I were him Jackson does have some training in sniper tactics and fieldcraft There is no evidence whatsoever that he is any sort of sharpshooter and until such sourceable evidence is provided it must remain as sniper which is the by far more accurate result in terms of both google search and reality  Son of the Defender