Thats tough to say. Guns alone dont decide history, if I could impliment modern telecommunications too? Now thats something different.
How different the world would be today if the Romans had undergone the industrial revolution a couple of hundred years after the birth of Christ. The technology and the information was there, they were at the same place technologicaly that Europe was at coming out of the Dark Ages...there is no Pre-Gunpowder nation or group of nations that could have beaten Rome at its height. Imagine the Industrial Revolution coming about in the third century instead of the ninteenth?
To make a truely impressive impact I'd take back a full expiditionary force to the first or second century AD. With us I'd bring a scientific library as well as individuals with basic know-how and skills with modern resource harvesting, chemistry, metulargy, physics, and earth sciences. With in a hundred years you could have electricity in every home, automobiles, aviation, navigation. With in a hundred and fifty, space-flight, the internet, modern chemistry and physics....
But...I'd only go back to introduce the information to everyone. That is only go back and introduce it to every major civilization around at the time. And I'd do my damndest to see that fissive nuclear physics never got off the ground, implement the widespread teaching of democracy and modern political theory, and environmental studies.
Think of a world with no 'third world' ? Where the standard of living everywhere is like the US or Western Europe? Where the environment is clean and well cared for ? (imagine hunting well managed heards of elephant, or buffalo in their natural environment ?) Where spaceflight has been around for a millenia ? Where all the children of the earth grow up with the knowledge of what it is to be truely free ? A full mind, a full belly, and the means to stand up for themselves both with ideals and with arms for what they believe ?
Knowledge is what I'd take back.