Howard Hill killed his buffulo from a bareback pony, as well as a number of cats and boar, with his usual 6' longbow. The shorter Plains style bows were, I belive I;ve read, developed mainly for war- as any bow they made oculd have been used to hunt with so long as it was strong enough.
They're little war bows could be shot very quickly and easily to wipe out white men and enemies without too much trouble. For hunting, they'd just need to get a couple arrows into the engine room at close range to drop a buffulo.