The two bullets I would recommend are the 250-grain Barnes TTSX and the 286 Nosler Partition. Haven't taken Cape buffalo with either, but when I hunted buffalo in Tanzania in 2011, my PH was so impressed with the way the 286 Partition worked on heavier plains game that he wanted me to use it on my second buffalo. ..
Should also mention that though technically the 9.3x62 isn't legal for buffalo in Tanzania, the government game scout that accompanies safaris agreed to let me use it, as he had also seen what it did. African caliber requirements are somewhat flexible, due to various factors.
In my tests both on various kinds of media and big game, the 286 Partition slightly out-penetrates the 250 TTSX, but the difference is slight. In your 20-inch barrel I'd be tempted to use the 250 TTSX, which modern--pressure handloads should result enough velocity to expand well. On the other hand, the 286 Partition's soft front core will definitely expand even when started at somewhat lower velocities.
I've taken both Cape Buff and NW Red buffalo with the 250 TSX and love it. I've seen my hunting partners have same results with it.
Let me add I had the same experience in Tanz. While "not legal", my PH and Game Guard had no issues with 9.3 x 62, especially after seeing early results.
Forgot to mention solids. In my experience, in both my own hunting and the buffalo my hunting partners have killed, solids aren't nearly as effective as expanding bullets, especially in smaller calibers like 9,3 and .375. But have even seen a buffalo shot several times all in the "boiler room," with modern 500-grain flat-nose solids from a .458 Lott that resulted in a long follow-up on a very hot day. The guy was actually hunting elephants instead of buffalo, the reason for only having solids, but they were far less effective than a good "soft" from a smaller cartridge.
Saw similar results with the 286 Barnes solid on Cape buffalo. It killed them, but they ran off in the thick stuff to die, waiting for a chance at revenge. One attacked a tracker and was put down before he could do any life threatening damage. My dad insisted on solids, he had a longer follow up than I did with TSX every time. Same rifle and well placed shots. Made me a TSX believer.
