It's not an argument. Just a question. Same pressure, same powder charge, everything. .260 rem with a 140 vs. .308 with a 140 loaded to the same pressure.
The problem with that is that it's impossible.
The same charge of the same powder will not result in the same pressure in two different cartridges, even two that are based on the same parent.
For instance....
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According to QuickLoad,
A .260Rem loaded with a 150gr Sierra MK would max out at 42.3gr Ramshot Hunter. Muzzle pressure is 9,577psi, 60k PSI peak, mv 2,677 from 24".
A .308Win loaded with a 150gr Sierra MK and 42.3gr Ramshot Hunter...
Muzzle pressure 6,126 psi, 25k psi peak, mv 2222.
So, they're not even close.
You can't even fit enough Ramshot Hunter in a .308 case to get up to 60k psi with a 150gr SMK.
If I go the other way, a 96% charge of Wiin748 in the .308 gets us to 60k psi peak, 7,853psi muzzle and 2,975 mv.
That same charge is a .260Rem...
122,000 psi (blow up the gun, or darn near), 9,587 psi muzzle and (a staggering) mv of 3,270.