All the calibers discussed here are good calibers.
But comparisons like this can become arbitrary and meaningless and might drive new shooters crazy.
I would start asking the question about the bullets wanted and speed on the target. Would the shooter like to have the option of a 160-180gr bullets and why?
The type of target or game when hunting, the weight of the game, the possibility of longer distances, all this is something only the new shooter can tell us and
this way decide what bullets are best for that job and what kind of margin for error one would feel confortable with.
The rest is a simple task of deciding what kind of casing I need to deliver the type of bullets I want where they are needed at the desired striking speeds.
Lets say we discarded the 30 caliber so let me give you a few examples with average type of loads to see how this could be relevant to the shooter.
Of course there are better bullets and more powerful rounds in any of those calibers.
http://www.hornady.com/store/270-Win-145-gr-ELD-X-Precision-Hunter/
Test Barrel (24") Velocity (fps) / Energy (ft-lbs)
MUZZLE 100 200 300 400 500
2970/2840 2796/2516 2627/2222 2465/1955
2306/1714 2157/1497
Trajectory (inches)
MUZZLE 100 200 300 400 500
-1.50 1.50 0.00 -6.50 -18.80 -37.60
Nosler...
http://load-data.nosler.com/load-data/270-winchester/
http://www.hornady.com/store/280-Remington-150-gr-ELD-X-Precision-Hunter/
Test Barrel (24") Velocity (fps) / Energy (ft-lbs)
MUZZLE 100 200 300 400 500
2925/2849 2763/2543 2607/2263 2455/2007
2309/1775 2167/1564
Trajectory (inches)
MUZZLE 100 200 300 400 500
-1.50 1.50 0.00 -6.70 -19.10 -38.00
Nosler...
http://load-data.nosler.com/load-data/280-remington/
The 7mm-08 comes less than 100 yards shorter than the 270 in terms of effectivness withing the critical 2200, 2100fps threshold.
http://www.hornady.com/store/7mm-08-Rem-150-gr-ELD-X-Precision-Hunter/
Test Barrel (24") Velocity (fps) / Energy (ft-lbs)
MUZZLE 100 200 300 400 500
2770/2555 2613/2274 2461/2018 2315/1784
2173/1573 2037/1381
Trajectory (inches)
MUZZLE 100 200 300 400 500
-1.50 1.80 0.00 -7.50 -21.60 -43.00
Nosler...
http://load-data.nosler.com/load-data/7mm-08-remington/
This is a rather mild load for the RemMag...
http://www.hornady.com/store/7mm-Rem-Mag-154-GR-InterBond-Superformance/
Test Barrel (24") Velocity (fps) / Energy (ft-lbs)
MUZZLE 100 200 300 400 500
3100/3286 2914/2904 2736/2560 2565/2250
2401/1970 2242/1718
Trajectory (inches)
MUZZLE 100 200 300 400 500
-1.50 1.30 0.00 -5.90 -17.20 -34.50
Nosler...
http://load-data.nosler.com/load-data/7mm-remington-magnum/
In 7mm you have the choice of 160-180grs with superior ballistics launched at very good speeds. You don't have that choice in 6.5 and limited in the 270.
In the 7mm you have the 7mm-08 small volume case choice combined with amazing 160gr-175gr bullets. You don't have that choice in the 6.5 or 270 bore.
You have the 284 variants and wildcats but then you also have the 284 straight that will produce better ballistics.
Of course we could add the WSM line but lets do this based on 30-06 and 308 siblings plus the popular remmag.
I included the 7mm-08 to show one could start with less powder and end with pretty decent effective range in case more is not needed.
Is there ammo that would outperform the above w/o getting into WSM, supermagnums or any other more exotic choices?
The most important question is, can I do more with less?
And when I say "do more" I don't mean just momentum and speed on the target but considering everything is important to the shooter including size, weight, recoil, expense, etc..