Hmmm...
I'd go with an AK or variant to the 1890s. A relatively simple rifle, with loads of potential for modification/ adaptation.
Scratch that.
I'd rather bring an old typewriter (once again, the most simplistic design imaginable) to the Romans. A culture with decent mathematical prowess, metalworking capability, etc. A culture with basically all the requirements for production of a cheap form of movable type. Ideas would spread further, for much cheaper, in greater quantity, advancing culture around the world at an amazing rate. It'd be about as revolutionary as the wheel, if not more.
In case that I'm not supposed to change history, I'd go back and meet my grandparents. Maybe go shooting with them.
Who says it ain't possible?