Actually, it isn't too diffcult to find a bullet that will penetrate a buffalo's chest from any reasonable angle and yet usually stay inside even on broadside shots.
One of the most common misconceptions in hunting is that a bullet that won't exit on a broadside shot behind the shoulder won't penetrate enough on an angling shot. But unless they disintegrate, almost all big game bullets are stopped by the hide on the far side, because it is elastic and tough. The same bullet isn't going to simpy stop in the middle of soft lung tissue somewhere in the middle of the animal.
As an example, I have seen quite a few elk and some moose taken with the 150-grain .270 Nosler Partition. It has usually stopped under the hide on the far side, even on rib shots, but I have also seen it penertate from the rear of a quartering away bull moose's ribs to the meat of the opposite shoulder--where it also ended up under the hide.
Many lead-cored bullets open widely enough and "rounded" enough to stop under the hide on the far side of a buffalo, especially if they open farily widely. Yet they will still penetrate on a frontal or angling shot.
Petal-type bullets, like the X, often pop through the hide, probably because the front end if less rounded. The petals may also do some cutting. At any rate, they seem to tear through the hide more than lead-cored bullets with more rounded from ends, such as the Swift A-Frame, Woodleigh or Nosler Partition. I once shot a buffalo with a 300-grain Fail Safe from a .375 H&H and the exit hole was a couple inches across--and one of the torn-off petals was found on the edge of the hole.
