A couple of posts back, someone asked Happy Myles what the best plains rifle was and Happy, with the guile born of much age and experience, never answered, thus avoiding the risk of being considered a gun writer. I, however, am beyond redemption, so I will answer, but first, a credential or two. I've been on ten safaris to 6 African countries, have taken most of the major trophies, and have brought along as my plains, or medium, rifle the following: .270 Winchester, .30/06, 7mm Weatherby, .300 Weatherby, .300 Win Mag, .338, and .375 H&H, some of these multiple times.
Plains game is an extremely loose catchall phrase that can take in anything from a 50-pound warthog to a 1,500-pound eland, with all stops in between. Indeed, that's part of the charm of Africa; you never know what's going to come boiling out of the bundu. Also, despite the fact that some species are no tougher to drop than whitetail deer, some of them don't go down without making an issue of it.
So, factoring that all in, the best plains rifle/cartridge I have used, in a walk, is the .338 Winchester Magnum. I started off with it in the 1980s using 250-grain Nosler Partitions, and while they worked flawlessly, I later switched to 225-grain Swift A-Frames and Barnes XXX all-copper bullets.
A 225-grain .338 bullet at 2,750 fps shoots as flat as a 180-grain .30/06 slug; you can hit with it with comparative ease at 300 yards, and most important, it provides Major Thump.
One of the realities of African hunting is that on occasion, you have to provide Major Thump. You can, if you wish, shoot a roan with a 7x57, but consider that a roan is 500-pounds plus, exceedingly tough, and the license fee for one is probably pushing $10,000 by now. If all does not go according to plan and you wound an animal and lose it, you still get to pay the ten grand. I, myself, would forswear the 7x57 for the .338.
A cool, precise shot can go to Africa with a deer rifle and do just fine. I know more than one person who has racked up a dozen plains game kills with that number of shots from a .270 or a 7mm/08 or something of that nature. But that's if everything goes to plan. If it doesn't, you'll wish you had a bigger gun. For me, the .338 is the ideal combination of maximum usable bullet velocity, bullet size, manageable recoil, and the ability to hit at longer (for Africa) ranges.
Is there a number two? In the opinion of this grizzled, embittered old observer, a .30/06 loaded with good, strong 180-grain (or better, 200-grain) bullets is a terrific choice. Recoil is much lighter, ammo is available anywhere for when the airline send yours to Vienna, and heaven knows the rifle will do the job.
I've had a .338 along at least three times as my plains rifle, and the only critter I ever had to shoot twice was a kudu that I hit badly on the first shot. Everything else went down forthwith. Better than that you cannot do.