Every chin can get shattered. But most often those extreme chinned guys are smart enough not to fall into an overhand like Pacman did.
Hagler was the closest I've ever seen to a chin that just seemed couldn't be shattered. Against Mugabi who was like 26-0 26KOs and a FEARSOME puncher, he literally ate ridiculous shots all night, and never even blinked. The shots he took from Hearns would have had any other fighter at that weight in la la land.
Gerald Mcclellan probably has a more ultimate chin. He had sparred with Hearns and according to Steward he walked right through a perfect right. And Hagler actually had a very good defence, only really a wild brawler against Hearns because that was the only way to beat Hearns.
McClellan didn't have an ungodly chin, and I've heard the same story, apparently he was only 16 when he did that. Nigel Benn was a fearsome puncher, McClellan was eating his shots without blinking once, until the horrific brain bleed occurred... One of the best chins I've ever seen for sure. Hagler did have a much better defense than people realize, but he was on the downslide vs Mugabi, and took MANY clean shots. Mugabi was a tremendous puncher, I really think he'd of knocked out anyone else in that division with those shots, he was snapping Haglers had back so far. Hagler never batted an eye, broke Mugabi's will, and knocked him out late.