How have the courts interpreted the definition of a sawed-off shotgun?

Relying upon Staples v. United States, 511 U.S. 600, 114 S.Ct. 1793, 128 L.Ed.2d 608 (1994), Keen next contends the district court erred by failing to instruct the jury that it needed to find that he knew of the characteristics of the sawed-off shotgun that subjected it to regulation under 26 U.S.C. 5861(d). The government initially responded to Keen's argument by contending that the rationale of Staples did not apply to sawed-off shotguns since the characteristics which render a sawed-off shotgun subject to regulation are readily ascertainable. See Staples, 511 U.S. at ----, 114 S.Ct. at 1800.