Law enforcement sources tell TMZ ... Ventura County Sheriff's deputies served a search warrant Monday at Simmon's home for illegal possession of firearms and ammunition. We're told cops detained Simmons as he drove away from his residence, and deputies found a loaded .45 caliber handgun in his car and more guns and ammo in his home.

Our sources say deputies discovered 14 firearms inside Simmons' residence, including assault rifles, a sawed-off shotgun and thousands of rounds of ammo.

We're told Simmons is a convicted felon, meaning he's prohibited from owning or possessing any firearms or ammo.

Our sources say Simmons was arrested on a slew of charges ... including felon in possession of a firearm, felon in possession of ammunition, possession of an assault weapon, carrying a concealed firearm within a vehicle, carrying a loaded firearm in a vehicle, unlawful transfer of a firearm, and possession of substance, material, or any combination with intent to make any destructive device/explosive.

We're told Narcotics and Gangs Units assisted Ventura County Sheriffs in serving the warrant, as well as California State Parks.