Quote from Wiki:
The Thompson Light Rifle was an attempt by Auto-Ordnance to manufacture a light rifle for the US Armed Forces. The overall weapon was based on their well proven .45 ACP submachine gun. Although the original .30 Carbine calibre platform was based on the M1921/27 variants, It worked well but due to the war effort was found expensive for mass production and defied the concept of a Light Rifle.The only major differences from the Light Rifle and SMG was the barrel shroud which housed a quick barrel change device similar to the MG42, pressed steel components to ease production/reduce weight and the inlined stock reduced barrel climb improving accuracy. The Thompson Light Rifle was more reliable and accurate than the M1 Carbine that was adopted and also came with the capability of select fire, which made it close to the likes of the StG-44