Federal law under 18 U.S.C. Section 924(c) imposes mandatory minimum sentences for using or possessing a firearm during a drug trafficking crime or crime of violence. The mandatory minimums are steep on their own: 5 years for possession, 7 years for brandishing, and 10 years for discharging the weapon. A second or subsequent conviction under 924(c) triggers a 25-year mandatory minimum on top of everything else.
The stacking problem comes from how these sentences are applied. Under the law, each 924(c) conviction must be served consecutively, meaning back-to-back, not only with each other but with any underlying sentence the person receives. A defendant charged with multiple counts of 924(c) in a single case can watch their total exposure grow into decades before a judge has any discretion in the matter. Judges cannot reduce these mandatory minimums regardless of the circumstances.
What makes the law particularly severe in practice is how broadly it gets applied. It can trigger on legally purchased registered firearms found in a person's home during a drug investigation, even if those guns were never present during the actual offense and never threatened or harmed anyone. A nonviolent first-time offender who had a legally owned rifle in their house while selling drugs out of it can face an extra five years stacked onto their sentence automatically.
The Armed Career Criminal Act adds another layer: anyone with three prior drug trafficking or violent felony convictions who is found in possession of a firearm or ammunition faces a 15-year mandatory minimum regardless of how old those priors are, how minor they were, or whether they ever resulted in prison time.
The First Step Act of 2018 narrowed the stacking rules somewhat for future cases, but many people sentenced under the old interpretation are still serving the results of those charges.