I've killed them with a 6x45, 6mm-204 and 243 and have hunted with guys with 6mm Rem and again 243. I don't know no of any 55gr to 100gr bullet short of FMJs and pure target bullets that don't work on coyotes. If your saving fur that's another story. I'd try them in a minute
Generally they are pretty stout as there are a number of 6mm Rem AIs, 240 Weatherbys and 6mm-06s out there and you don't hear much about bullet failures as far as velocity destroying 6mm bullets.