Well, I think you're not quite seeing one of the main draws of the Fate system, which is the level of abstraction that it offers. We don't need to have long, detailed discussions about what gun is the best in which situation, because they're all more or less equal. There's no way to differentiate them mechanically, so it becomes more a matter of which is cinematically more suited to your character or the situation.
Although I'd argue that a big game hunting rifle would allow you to throw a maneuver on a target of an "In My Sights", which is practically +2 damage right there. You could possibly put something on it along the lines of Weapon:3 (Weapon:4 against Hulking Size opponents) as well, but that is veering strictly away from the main draw of Fate, that being the enormous level of abstraction.
Baaaaasically, I think what you want to do is write a whole new guns and weapons supplement for Fate using your own versions of their rules. Where you set the granularity finer so you can make distinctions like that. I'm not sure how well you'll be able to do that without either making skills more or less obsolete and guns/weapons dominant or make people/things much more durable. Basically, as it stands now, unless someone has some level of special, you can take them out with a single shot from most guns. Hell, even a rhino is pretty fair pickings for an instant or maybe two-round take down. Weapon:3, In My Sights, Roll of +2 or better, and unless he takes a consequence of some sort, he's taken out. And what normal rhino would think to take a consequence?