In 2010, an explosion at Black Mag in New Hampshire killed two workers.
During the blast, Jesse Kennett, 49, and Donald Kendall, 56, were operating machines at the plant, which manufactured a black powder substitute for muzzleloading rifles.
Company owner Craig Sanborn was prosecuted for ignoring safety measures and improperly training his staff.
In 2013, Sanborn was convicted of two counts of manslaughter and sentenced to 10 to 20 years in a state prison.
Last week, Sanborn, 71, was denied early release from prison.
After serving 80 months of his 120-month sentence, Sanborn was able to ask the court to suspend the remaining sentence.
Sanborn will be 74 when he reaches his minimum release date in November 2023.
While Sanborn has been a model inmate and a mentor to others during his incarceration, he still faces 28 months in federal prison for wire fraud.