In the Army it’s the 11 series Infantry , some 12 series, Combat Engineers, 13 series Field Artillery and 19 series Armored Armored Crewmen. Some 18 series Special Forces Operator, but they are too highly trained to be a grunt. In combat engineers if you carry detonation cord and a chainsaw or
do road clearance you’re a grunt, if you drove a bulldozer or build schools you’re not a grunt. Young infantry noobs try to claim grunt for their own but
old timers know it’s almost all combat arms. Army Aviators are combat Arms also, but while the job can be incredibly dangerous the frequency of hot showers and contracted chow available at airfields means no grunt for you.
19k Cavalry scout used to be Grunts when the army had Armored Cavalry Regiments , but since those were transformed into Cavalry in name only infantry regiments the cav scouts grunt status is up for review.