At the murder scene, at Tony's food truck, Carlos extracts a bullet from the back wall with a pair of forceps, looks at it, and immediately identifies it as a 9mm. There is only, at most, a two-one thousandths difference between 9mm, .38, .380, and .357 bullets, and it would take a precise measurement with a micrometer as well as precise weight measurements, to determine the specific caliber of the bullet.
When Jim serves Vanessa Russo with the search warrant, it states that it was issued by the United States Circuit Court. United States District and Circuit Courts only have jurisdiction over federal crimes, and the homicide in question would be a state crime; the warrant would have been issued by a Florida state circuit court.