Andrew Kaufman, 36, and Omar Aguirre, 35, appeared for the plea deal before Senior U.S. District Court Judge Lloyd George in Las Vegas, U.S. Attorney Daniel Bogden said in a statement.
Aguirre wasn't previously named after a monthslong undercover operation became public with the Nov. 3 arrests of Kaufman in Las Vegas; active-duty Special Warfare Operator 1st Class Nicholas Bickle, 33, in San Diego; and Bickle's friend Richard Paul, 34, in Durango, Colo.
Bickle and Paul pleaded not guilty Thursday in Las Vegas to amended conspiracy, weapons, explosives, firearms trafficking, weapon registration and dealing in firearms without a license charges, said Natalie Collins, spokeswoman for Bogden.
The judge allowed both men to remain free pending trial Jan. 24.
Kaufman could face up to 15 years in prison for his guilty plea to conspiracy and illegal transfer of a machine gun, according to a plea agreement. Aguirre faces up to five years for conspiracy. Sentencing for both men is scheduled for March 25.
Both are also expected to testify against Bickle and Paul, who could face decades in federal prison if convicted.
Prosecutors have described Bickle as the mastermind and chief weapons smuggler and Paul as the primary seller in a scheme that sold handguns and military machine guns to an undercover federal Bureau of Alcohol, Tobacco, Firearms and Explosives agent who told them they would be shipped to Mexico.
Federal agents also seized some five pounds of C-4 military explosives from Paul's Colorado home following his arrest, authorities said.
Bickle's attorney, James Pokorny of San Diego, said he'll ask a judge next week to allow more time before trial so he can review government evidence.
"The case is still in its infancy," Pokorny said. "A lot more will be revealed as this case goes forward."
Paul's federal public defender didn't immediately respond late Thursday to messages seeking comment.
According to the Kaufman and Aguirre plea memorandums, more than 70 firearms were smuggled and sold, including rifles, pistols, shotguns, and 30 machine guns.
Federal prosecutor Drew Smith told a judge in November that some of the AK-47 rifles included factory-made 7.62 mm Iraqi machine guns that would be difficult or impossible to trace.
Court documents said the scheme was made possible because Navy SEALs aren't searched when returning from deployments.