For example, AIM-120 AMRAAM, Tomahawk Cruise Missile, or Harpoon anti-ship missile. They fly straight and level for the most part of their flight regime. I think if they had a fuselage optimised for level flight then they could have a greater range. Something of a lifting-fuselage.

I know that even their cylindrical body generates lift at some angle of attack but it comes at the expense of drag, so a more optimised shape could result in a greater L/D ratio and hence a longer range.

I think that space and volume constraints may be an issue, like a circular cross section probably has the largest volume to store whatever.