Former Blackwater guards Paul Slough, right, and Nick Slatten leave federal court in Salt Lake City, Utah, on Dec. 8, 2008. (Douglas C. Pizac/AP)

Three of four Blackwater Worldwide guards scheduled to be sentenced next month for a 2007 shooting that killed or wounded 31 unarmed Iraqis asked a federal judge for leniency Monday, saying a mandatory minimum prison sentence of 30 years is unconstitutionally harsh.

Paul A. Slough of Keller, Tex.; Evan S. Liberty of Rochester, N.H.; and Dustin L. Heard of Knoxville, Tenn., were found guilty in October of multiple counts of manslaughter and attempted manslaughter in the Sept. 16, 2007, incident at Baghdad’s Nisour Square.

The men face enhanced penalties at sentencing before U.S. District Judge Royce C. Lamberth on April 13 because the same District federal jury also convicted them of using military firearms while committing a felony.

Attorneys for the men said that the U.S. government has never before prosecuted its own security contractors for the firearms violation, noting that in this case, the government gave their clients the weapons they used in their missions.

A 13-page defense motion argued that a 30-year sentence would result in “grossly disproportionate” and “cruel and unusual” punishment, given the “unique” and “unprecedented circumstances.”

A fourth man, Nicholas A. Slatten of Sparta, Tenn., was convicted of murder and faces a mandatory sentence of life in prison. All four men are veterans.