Mick is correct, of course, as he has seen many buffalo taken by others as well as having shot them himself. I shot mine with a 68#@29.5" Super Shrew Samurai and will be using a 70#@29.5" Super Shrew Safari on my next buff hunt with Mick and Andy in July. I used AD Hammerhead arrows with 2 blade heads for a total weight of about 900 grains. After my buff was dead, I shot it several times to test penetration from various angles. I can tell you that on my huge buffalo (100" horns) penetration was very problematic except on broadside shots directly into the meaty portion of the shoulder behind the leg bone. On a smaller animal I might have been able to split the ribs, and perhaps could have done so on my buff with a 90# bow and 1200 grain arrows, but at my age, with the injuries and subsequent surgery that my shoulder has undergone, 70# is the maximum draw weight I can shoot. That is certainly enough; however, if I could draw more poundage, I definitely would use it.
As others here have said, it is the marginal shots where the extra weight of both the bow and the arrow can make a big difference. Andy Ivy has used both 2 and 3 blade heads with comparable results on broadside shoulder hits. I tend to favor 2 blade heads, but any head has to be super sharp and super strong. I would recommend the 300 grain VPA heads (2 or 3 blade), although I have heard great reports on the Tuffheads too, and heads like the Silver Flame, STOS, ABS Ashby, and Grizzly have had success in the past. I have seen chipping on some of the single bevel heads, like the ones I used, and believe the forces that help twist the head and split bone may also be helping to chip the blades. There may also be some metallurgy issues with various blades. Some heads also tend to break at the front of the ferrule, which is very bad for penetration.
The ribs on my buffalo were about the size of a man's femur, so they are very difficult to penetrate or break, regardless of what some folks may think after reading Ed Ashby's excellent reports (read them carefully). Remember that deer, elk, moose, and most other game animals are in no way comparable to the really big and tough critters like buffalo, rhino, croc, hippo and elephant. For those animals you need equipment of a much heavier, sturdier level unless everything is absolutely perfect -- which it almost never is.
Allan