What is the burden of raising an affirmative defense?

1 The statutory exception created by 922(o)(2)(B) is an affirmative defense, and the defendant therefore bears the burden of raising it. See United States v. Gonzales, 121 F.3d 928, 936-37 (5th Cir.1997), cert. denied, --- U.S. ----, 118 S.Ct. 726, 139 L.Ed.2d 664 (1998); United States v. Gravenmeir, 121 F.3d 526, 528 (9th Cir.1997); United States v. Just, 74 F.3d 902, 904 (8th Cir.1996); see also United States v. Mayo, 705 F.2d 62, 74 (2d Cir.1983) (noting that defendant has burden of raising affirmative defense)