There is a rather well-known alternate history novel called Guns of the South in which time-traveling South African racists supply RE Lee with large numbers of AKs plus ammo. Which as you might imagine, results in a Confederate victory.
Yep, a Harry Turtledove yarn. There's a less well-known Science Fiction alternative history in which the postwar US cavalry is better equipped with repeating carbines, but the Aboriginies have developed heavier-than-air aircraft.
And then there's my twisted history, in which General George Patton, about to return to the US after being relieved of command of Third Army, hears the news of the crash of Ike Eisenhower's transport plane, with most of his headquarters staff aboard. That changes things around just a tad.