One of the detectives reports that a subway worker found a "25 mm handgun on the subway tracks." The gun is actually .25 caliber, specifically a .25 Automatic. The bullet for a .25 Auto is .251 inches in diameter, or 6.4 mm. A 25 mm bullet would be .98 inches in diameter, only guns that large would be found on a tank or an antiaircraft gun on a frigate or a destroyer.
There is no way to discern the type of gun used based only on a recovered bullet.
When E.A.D.A. Jack McCoy visits Father Paul to discuss Father Evans' predicament, at first he does not mention to Father Paul anything about a murder, just that Father Evans was apparently withholding evidence of a "crime." Father Paul claims no priest would "absolve a murderer" without actually being told that a homicide has occurred.