Big grizzlies have been known to take a half-dozen, or more, heavy rifle rounds into the lungs and heart and still wreak havoc on the shooter(s). Yes, 00 buck packs a wallop, but it is still very marginal for big grizzley and you will not find many experienced hunters recommending it as the best solution. 00 buck's power is at relatively close range, and if that bear is coming, you'd better be able to get it all done in about 5 seconds, literally, or you're probably going to be dead while you're still wondering why he didn't stop. He'll kill you easily if he gets to you. It's gonna depend on the situation, though, and how high the adrenaline is running in the bear. If he gets focused on you and decides he wants to kill you, 00 buck is going to be iffy. Very iffy. My experience is with African game...which most agree is generally tougher to kill. But I suspect a big male grizzley with his adrenaline up is comparable to a lion in his desire to "get there" and I've seen, first hand, what a lion can still do, and how far he can come, with 3 .375s in him, any one of which was shortly going to be a killing shot. At that point, you realize how relative "shortly" can be.