Police in Colorado Springs used 120 rounds of rifle fire to kill five Buffaloes that had escaped from a meat packing plant. The officers were using .223 caliber weapons, which are just fine for killing people, but pretty inadequate for a 900 pound Buffalo. .223 ammunition is not even allowed for deer hunting in some states. For reference, 19th century Buffalo Rifles were traditionally chambered for calibers in the .45 to .50 range, more than twice the size of the weapons used. Of course, Kalifornia, New Jersey, and some other states are banning .50 caliber weapons.
There is some question as to the need to shoot them at all. The director of the trucking company says that he has always been able to coax escaped animals back into the pen. One article also mentions that the animals were shot one-by-one as the charged the officers. If I was getting shot at with a pea-shooter (relatively speaking), I might charge my assailant too!