The ACCA, codified at 18 U.S.C. § 924(e), imposes a 15-year mandatory minimum sentence on defendants convicted of being a felon in possession of a firearm who have three or more prior convictions for 'violent felonies' or 'serious drug offenses.' Without ACCA, the maximum sentence for felon-in-possession is typically 10 years.

Under the ACCA, a violent felony is defined as a crime punishable by more than one year that: (1) has as an element the use, attempted use, or threatened use of physical force against another person (the 'elements clause'), or (2) is burglary, arson, extortion, or involves the use of explosives (the 'enumerated offenses clause'). The former 'residual clause' was struck down as unconstitutionally vague by the Supreme Court in Johnson v. United States (2015).

The categorical approach, required by the Supreme Court in Taylor v. United States, compares the elements of the prior conviction statute to the generic federal definition of the offense. If the state statute is broader than the generic definition, the conviction may not count as an ACCA predicate. This analysis can be complex and is often where experienced defense attorneys find successful challenges.

Yes. Common challenges include arguing that prior convictions don't meet the categorical definition of violent felony or serious drug offense, demonstrating that convictions were not committed on 'occasions different from one another,' challenging whether the conviction was final, and using the modified categorical approach to show the actual conduct doesn't qualify.

In Johnson v. United States (2015), the Supreme Court struck down the ACCA's residual clause as unconstitutionally vague. This means prior convictions that only qualified under the residual clause can no longer count as ACCA predicates, potentially making some defendants no longer subject to the 15-year mandatory minimum. Many federal prisoners have sought resentencing based on this decision.