Ordinarily, a judgment dismissing a case on the ground that the court lacks subject matter jurisdiction over is not a judgment on the merits and does not operate as res judicata.  However, in the unique context of the Federal Tort Claims Act, the court has subject matter jurisdiction only if the complaint plausibly alleges all six elements of an FTCA claim.  So the 12(b)(1) and 12(b)(6) concerns totally overlap.  As a result, a judgment dismissing an FTCA claim on the ground of lack of subject matter jurisdiction operates as res judicata, and under 28 U.S.C. § 2676 also bars a claim on the same facts against any federal employee involved in the incident.