Are you sure that a previous owner didn't simply have a nickel 4", wanted a 2.5", and all he could get was a 66 barrel? That's a lot more likely than a cracked forcing cone!
Okay. I haven't seen that many of them so can't say which failed more often.Well, that's the first cracked 66 I have heard about. But trust me, the 19-5 is the one that fails most often.