The biggest confusion in the shooting community regarding flat shooting is external ballistics vs external bull-istics.
Like Cat said earlier. When I hear flat shooting I think a cartridges ability to stretch the Max point blank range. This allows for a more forgiving shot when rushed and you need to make a quick decision and don't have the luxury doping your shot. That’s the first thing that comes to my mind. Although, if you’re asking for a definition of what flat shooting means. I can’t provide that. There are too many ideas out there and not a straight definition. Many of these wicked fast varmint cartridges are great up close (under 4-500 yards) but past that range, they drop like a rock and are not so FLAT shooting anymore. So are they still classified as flat shooting? That’s kind of what I’m getting at and I believe the OP is asking too. You can’t compare apples to oranges. It’s the individual who does compare apples to oranges, these are the same individuals who come up with these outlandish stories or state completely bassackwards information.
It's funny though. I've spoken with a few individuals who are self-proclaimed experts as well. Most everyone knows or agree to some extent that the force of gravity and air resistance are the main forces the affects a bullets rate of fall. It’s when the speed of a certain bullet vs another bullet is discussed that those people get all confused.
When comparing bullet A to Bullet A, then yes speed is the only factor that will affect the ‘flat’ trajectory (or max point blank range) that most speak of. That’s because those bullets are identical. Being identical, then theoretically speaking; regardless of one travelling at 0 fps away from the gun and the other at a blistering speed of 5000 fps. They will both inevitably rest in the dirt at the same time. I find that the people who tell me that there gun shoots so flat that they can shoot at deer at 500 yards without a holdover (of course I’m using this as an exaggerated example) are the individuals who originally agreed with the above statement that gravity and air resistance affect a bullet, ect. They agree with the statement but don’t understand the ‘air resistance part’. All they know is that light and fast is best! And if you’re not shooting at long ranges then that theory will probably serve you well. These individuals are the same ones who state ‘lighter bullets are affected by the wind more than heavier bullets’. They say that because they don’t understand that weight has nothing to do with it. Generally speaking, yes lighter bullets are affected more in the wind more than heavier ones. Although, that’s not because of their weight.
Ie, compare the lighter 105gr .243 cal bullet to a heavy 230gr .45cal bullet. Both shot at the same velocity, even though the one bullet is more than twice the weight of the first it will be affected by the wind more than the first. That’s because of the Ballistic coefficient. These bullets are kind of like comparing apples to oranges though. It’s just an example. The first is designed for rifle cartridge and the second is designed for a pistol cartridge.
Back to self-proclaimed experts. It’s when these people forget about or don’t understand air resistance that the problem occurs. These individuals get confused and proclaim useless exaggerated claims when they start to compare cartridges against others. And when doing so unintentionally compare apples to oranges.
If you’re really interested in learning about what effects the external ballistics of your bullet then I’d recommend reading a lot of literature on physics or literature from rifle enthusiasts point of view but backed with numbers. There is a ton of great information out there.
Here’s a site that really opened my eyes when I first started t to learn about projectiles and external ballistics.
http://www.frfrogspad.com/extbal.htm