The smooth edges and ridges around the circumference lead me to believe that it is something that is meant to be held by one's hand, so maybe the beginnings of a smoke grenade?
Since it is round all the way, it probably isn't anything big. Nobody would design a large canister that would roll away so easily. At least, it isn't anything that is meant to sit around for a while.
It definitely has a canister shape, though. The flat edges of the rings around the circumfrence on the near side aren't enough to keep it from rolling away because they are lower than the single ridge that spans the length, but it might be designed to be put up against something. Maybe it is a fuel tank for a jet (wings), truck (underside), or tank (back)? Or a sort of munitions canister, like for a cluster bomb, meant to split while falling.