people say the .460 is flat shooting, whereas the 45-70 is not.
Probably because they are comparing the .460 in its natural state, as a "hyper" velocity PISTOL cartridge (2200fps+) and the .45-70 in its standard 1873 blackpowder velocity loading (1300fps).
If you compare bullets of identical weight at the same velocities, one a .452" (.460S&W) and the other a .458" (.45-70) there is no significant different in their trajectories, other than the small amount created by differences in their BC due to their nose profiles.
The .460 shoots "very flat" for a PISTOL ROUND, by current standards.
The .45-70 does not shoot "very flat" for a RIFLE ROUND, by current standards.
As you say 300gn bullets at 2400fps (2250 being the velocity from the XVR revolvers) would kick, but less than a 450gn bullet in the 45-70.
Recoil energy is a straight computation. There is a point where the 300gr and the 450gr bullets produce exactly the same amount of recoil energy. However, at that point, the lighter bullet is moving much faster than the heavier one. Move them both at the same speed, the heavier slug will always have the higher recoil energy.
How we each perceive that energy is called "felt recoil" and is entirely subjective, a matter of what each different shooter "feels" when they fire the gun. This involves numerous "shooter" factors beyond the gun and the ammo themselves. How the gun "fits" the shooter, how the shooter "fits the gun" (meaning how they hold and fire it) and other factors make a difference in the feel for each individual.
For example, a friend of mine thinks my Ruger No.3 .45-70 "kicks like a mule" shooting the standard 405gr load, and refuses to shoot it with anything hotter.
(He shoots Sharps & Winchester .45-70s, considerably heavier rifles with different stock dimensions.
I think the recoil from my No.3 with standard loads is a moderate shove, and not at all disagreeable. Raise that same bullet to 1800+fps and the recoil changes HUGELY! And the lighter the rifle, the more...noticeable it gets!
When I had my .460PC with a 10.5" barrel I was able to push 300gn bullets to 2100fps. That equates to almost 2900lbs of muzzle energy which is more than a .30-06. Thankfully the gun with scope, 3 rings, and bi-pod was pushing 9 lbs which helped absorb some of the recoil. I couldn't imagine how it would feel in a 7lb lever gun.
A "pushing" 9lb handgun! WOW!!
I realize how it gets there, and how the weight helps with the recoil, I just can't see how a handgun that heavy is any real advantage to a rifle, other than being shorter.
I have a 350gr load that clocks 2200fps from my Ruger No.3, which is a
6lb rifle! This load is a "walk you back one step to keep from falling" level beast when fired offhand, and in the never, never, NEVER fire from the prone position group in my book. So is the 400gr at 1800fps from this light carbine.
AND these loads are much lower pressure than the 65K psi of the .460 S&W!