Some weapons I think that can be added to the United States that are unique to them, and branch out from some that the British Empire currently have like the M1918 BAR, Trench Shotgun, and 1911, and others:
• American M1918 Chauchat, designed to improve the Chauchat’s flaws and meet their needs due to the BAR not being widely available until the end of the war.
• Annihilator Thompson, though never seeing combat, a unique SMG that can be given to US stormtroopers
• M1903 Springfield, standard rifle of the US armed forces
• Winchester 1907, a civilian semi-automatic hunting rifle that was adapted to trench warfare for military use.
• Colt New Service, a revolver from 1898 that was the common service pistol of the US armed forces before the 1911.
Now, there is a very few unique American weapons of that time, as many of what the US had were also British and french weapons at that time since the United States wasn’t even a grand military power just yet.