Dist. Ct. erred in finding that defendant’s prior Wisc. conviction on charge of possession of short-barreled shotgun qualified as “violent felony” under residual clause of Armed Career Criminal Act that required Dist. Ct. to impose at least mandatory minimum sentence of 15 years imprisonment on charge of felon in possession of firearm. Defendant’s mere possession of short-barreled shotgun was not similar to offenses enumerated in section 924(e)(2)(B)(ii) as guides to evaluate nature and degree of risk of injury since passive possession of such firearm poses no immediate risk of harm to anyone.