Prosecutors say 66-year-old Karl Twilleager, or "King Karl," is the leader of the Ghost Riders motorcycle gang.
The Spanaway man has been in jail since June, when ATF agents found two sticks of emulsion explosives and four blasting caps in a storage locker belonging to Twilleager.
The search also turned up three rifles, two shotguns, a handgun and ammunition, all which was illegal because Twilleager is a convicted murderer who cannot own guns or explosives.
Two days later, agents searched Twilleager's house and a shed on his property that was used as a Ghost Riders clubhouse. They found six more guns and multiple rounds of ammunition, according to prosecutors.
On Thursday, Twilleager pleaded guilty to possessing an unregistered short-barreled shotgun and being a felon in possession of explosives, according to prosecutors.
When he's sentenced in December, prosecutors and the defense will jointly recommend a five year prison term. However, the judge may impose any sentence up to the ten year maximum allowed by law.