The only time they would wear a combat uniform such as a S.W.A.T. type you are talking about would be for a high risk warrent or assualt like the Karresh raid back in the early 90's. Mostly that kind of thing would be left to the ATF, DEA and FBI. The U.S. Marshals are more detective and investigators. Now you have the whole Federal Prisoner transport unit and they were uniforms like that but they are a much smaller group.