I have been tutoring a couple of guys in LR hunting for a while, not that they need much of my help anymore, very fast learners, very keen. One guy shoots a .300 Win Mag, the other shoots a 7mm Rem Mag. Both cartridges have performed exceptionally well and the guys are currently obtaining clean one shot kills/ good groupings at 700 yards which is a couple of hundred yards beyond the 500 yards they anticiapated being able to shoot to.
The hunter with the 7mmRM has put 1000 rounds through his barrel since last August. When its time to re-barrel, he is thinking about doing the 7mm Rem mag again but withy a longer barrel so that he can shoot the 180 grain VLD at a comfortable velocity.
My wife has a 24" barreled 7mm Rem mag. Velocity with the sleek 180 grain VLD is 2900fps. For every inch of barrel added, you would obtain another 35fps. With a 28" barrel, 3000fps would be quite obtainable with plenty of room for obtaining the right load (not having to push it).
As for any differences between the 300 and 7mm. At the ranges of 500 to 700 yards, I can't tell the difference between the two. Both perform well and success comes down to whoever can read the wind the best.If you shoot a lot of tougher animals or game weighing over 200lb, the .300 will give you a little more room to play with heavier bullets or simply 180 grain controlled expanding bullets.