This is a war movie about a group of men being chasing by an Apache group that is using spears and arrows with horses. They end up running to the secret cave... then they decide to go in the cave and found a modern weapon which is like an M-16 grenade, etc that shouldn't exist in that era. They start to learn to use the modern weapon that is found in this mysterious secret cave to fight back.
The mix of Cowboys and high technology in the 1970’s TV show Cliffhangers? Being chased, stumbling into a cave, and finding weapons.
The Secret Empire – Set in the 1880 and inspired by the 1935 Gene Autry film serial The Phantom Empire, Geoffrey Rush would star as U.S. Marshal Jim Donner who, while investigating a mysterious band of phantom riders, stumbles upon an alien city deep beneath the Earth. Scenes in the subterranean city would be broadcast in color while the above ground scenes featured a sepia tone.