To answer your question: A 358WSM 1.8" (BFG or otherwise), shooting a suitable bullet that will open up properly at the distance you'll be hunting, is certainly the flattest-shooting, longest-range option available.
Well, kinda. You are forgetting anything over .243" can be shot from a handgun. Now, I would think that a .243" bullet, from a 15 15/16" barrel, loaded to the maximum allowable by SAAMI standards would be much flatter shooting than our big "punkin" balls.
You just have to shoot it from a handgun and not a rifle is all!
Just to be perfectly clear.