Weight retention isn't a big deal when using copper. This is not a feature unique to Barnes. However, they do make the disclaimer that weight retention is at normal impact velocities. At short ranges and hitting hard tissue such as bone, they note that their bullets certainly can fail and lose petals.
Taurus solid copper bullets also have the same claim of virtually 100% weight retention as well.
Also see
http://www.chuckhawks.com/barnes_bul...zzleloader.htm