What size bullet does a 50cal take?

If you mean a 50 caliber gun, it takes a 50 caliber bullet, but there are a couple of different kinds. Are you referring to a rifle or a handgun? If you are talking about a handgun like a desert eagle then it is a 12.7x33 in metric, .50AE standard. If you are talking about a rifle then it can be 12.7x99 in metric, .50BMG standard used in rifles like the M-82 or it can be a 12.7x108 metric used in rifles like the Gepard M-1. Also the bore in all the .50's is really .511.