Wagyu raised in the United States is not professionally rated as it is in Japan (a skill that requires years of training.) Hence, there is no official BMS (or Beef Marbling Standard) designation for USA-raised Wagyu.
That said, even the Fullblood (100% by DNA pure Wagyu) raised in the USA would not achieve the highest BMS scores of their Japanese namesakes. And the Angus-Wagyu crossbred wagyu will be even lower in marbling, even though it will typical grade well above USDA Prime.
F1 = wagyu-angus cross (first generation)
F2 = second generation
In all, if it was rated, most American Wagyu would typically end up grading at a respectable BMS score of between 4 and 7, depending on the farm, season and other variables.
Read The Ultimate Guide to Wagyu on our blog for more information.