Was the M3 carbine used on Okinawa? Yes, as the T3 carbine with the T120 sniperscope. They would later be classified as the M3 carbine with the M1 sniperscope.

[Note: pictured is a T3 carbine with an M2 sniperscope]

While it had a scope, it wasn't a sniper weapon. The infrared scope had an effective range of about 70 meters which matched well with the short range of the M1 carbine it was derived from. The M3 was primarily used to great effect to defend against Japanese nighttime infiltration.

Was the 1st Marine Division issued any T3/T120s? I don't know, but I doubt it. It was an Army project and only about 150 were on Okinawa. I doubt they gave any to the Marines.

Sources

• Forgotten Weapons: M3 Infrared Sniper Carbine at RIA
• Rock Island Auction Company: Lot 1788: Inland T3 Carbine 30 M1 Carbine with "Repair Instructions for Sniperscopes M1 and M2" August 1951
• The U.S. Carbine Caliber .30 Model M1E7 & Model T3