The Olympian reports that Spc. Thomas Patrick Popek was arraigned in court Monday on an assault charge.
The 23-year-old victim from Hudson, New Hampshire, is in critical condition and unable to breath on his own.
Court documents say the shooting took place Saturday in a Parkland apartment, about 40 miles south of Seattle.
Popek reportedly told investigators he had been playing around with his gun while cleaning the weapon when he pointed it at the victim and pulled the trigger.
He says he didn't know the gun was loaded.
The two men had been preparing to go to a shooting range at the time.