Narrowing the choices.
Anything post WWI, my personal stuff wouldn't be of any technological advantage. Bolt and semi auto rifles, pump shotguns, semiauto handguns. So I'll look at pre-1910.
Playing devils' advocate, it might be interesting to see what would have happened if the "locals" (indians, first nations, whatever name you know them by) had possesed superior firepower from the git-go. Americas' westward expension might have stopped at the Mississippi.
The greatest impact could have been made in the revolution, however. Imagine being on the bridge one fine April morning with an AR. Several hundred rounds pre-loaded in mags, the entire british force marching in formation. Or even a scoped .308. You could be making killing shots before you were even in range of volley fire. Once they finally closed, a handgun makes a much better weapon than a bayonet or cutlass.
Going a bit south, giving the Maya and Aztec repeating arms would have drastically altered the course of settlement in the Americas.