One thing that seems pretty clear for the US Army in TL-191 is their arsenal of small arms. Its explicitly stated numerous times that the Army uses "Springfields", "Thompsons", and ".50 cals". From the sound of it they seem to be our timeline's very own M1903 Springfield, M1A1 Thompson, and Browning M2HB. I believe at one point a "carbine" was mentioned too, likely the M1 Carbine, but I could be wrong.
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So we're looking at an arsenal here that might be roughly comparable to our timeline's US WWII arsenal. However, we'd be missing an iconic and important weapon - the M1 Garand. The Garand is not mentioned
at all in TL-191 and its absence is sorely felt by US soldiers. The Springfield is a good rifle, a reliable one at that, proven in the trenches of the Great War. By the Second Great War, it is outclassed by the TAR and US soldiers feel the terrible gap in firepower.
I think that's a pretty interesting butterfly effect here. As for the why the US Military didn't develop its own semi-auto or full-auto rifle we might never truly know. Either way, unlike the Confederates, the weapons used by the US Army are comfortably familiar.