Does the 7mm Remington Magnum perform best with a 24" or 26" barrel?
All things being equal(they rarely are)a 26" barrel in 7 Rem Mag should deliver higher velocity than a 24". But velocity can vary as much between two different 24" barrels as it will between a 24 and a 26",depending on make of barrel, throat,etc etc.
Out of numerous 7 Rem Mags, all mine have had 24" barrels,except for a couple of M70 Classics,which had 26". They did not hang around long at that length.
Hard to generalize,and there are many variables that crop up, but these are the velocities I have seen with hand loads in 24" 7 Rem Mags:
175's- about 2900 fps.
160's- 3000-3100 fps
150's- 3150 fps
140's 3200-3300 fps.