Oscar,
There is a good book called 'The Perfect Shot' by Kevin Robertson that might help you, and I think it is available on the Internet for about $65.

I borrowed it to read and, as I recall, the best shots on buffalo (and other ungulates) tend to be:

Side on

1. the top of the heart (about four-tenths of the way up the chest, in line with the front leg, at the top point of the shoulder visible under the skin), which will also pierce the lungs.

2. the spine where it dips sharply down near the front of the lungs, slightly above the level of the heart shot (1). A miss behind the spine should at least get the lungs.

From the front

1. just beneath the chin to hit the spine at the bottom of its dip in front of the shoulder.

2. the brain, in the middle of the forehead above the eyes; if the buffalo is charging with its head up, this may require a shot up or along the nose.

3. top of the heart, possibly a fraction below the middle of the chest.

In writing this stuff, I realise an explanation without Robertson's pictures is inadequate. Get the book and read about all the other considerations required as well.

Cheers
- Paul