I have not seen calves, but have talked with Jay, who owns them. He said they have about 8 on the ground so far, and birth weights have been from the lower 70's to the lower 90's, with heaviest one being two weeks late. Calves are big topped, big hipped and stylish. He said you should be safe to use him on heifers as long as the heifers are stout hipped enough to have a decent sized calf. Combustible is a very stout bull so he wouldn't expect him to throw 60 lb calves.
Hope that helps. You can just ask Jay yourself. We plan on using him on a few heifers, but we watch our girls closely.