Why? Do you plan on being home-invaded by a pit bull?
Seriously, you're learning the wrong lesson here. The OP shot a pit bull- a heavily muscled, hide-covered, thick-boned, aggressive beast- and shot it several times in non-incapacitating areas. He then shot it a final time in an area that can and did incapacitate the dog. From this, you're gathering that the load used would be ineffective against thin-skinned, lightly muscled, thin-boned humans. This does not compute.
Upgrading from "low recoil" to full power loads might be a good idea, but its status as good or bad has nothing to do with this incident. What I take from this, unless I am misreading it, is that once the OP got a solid hit, even with RR 00, the dog stopped right there. That's still a good performance for the 00, in my opinion.
That said, the obvious lesson is that the 12g is not, as was said, a death ray. You still have to hit. You still have to hit important stuff. And, even then, it might not kill. It might not even stop, but that's the way I'd bet.
BTW, the OP seems to have done a good job. It's amazingly hard to hit a charging dog. The last dog shoot I saw involved 3 rounds of Federal RR 00 buckshot being fired in about 1 second. Range was 30' and closing, FAST! The dog was hit by multiple pellets, not sure how many times, but was not hit in a vital area. The
coup de grace was a administered at a range of about 5', by a .40 S&W Federal HST round, through the throat, fired by a second officer.
He drank for free that week.
Mike