For long range gongs, use the brinell 500 plate. To prevent a similar incident from the guy in the .50BMG video, use an "L"shaped hanger bracket. With the weight of the plate hanging from it, it will slightly angle the plate to deflect the round into the dirt.
For silhouette pistol targets, use brinell 400. Make the plate channel at an angle so the top of the target leans towards the shooter. Again, this deflects the round into the ground.
For a falling plate rack, use the 400 again and mount the plates on a common shaft. A cam on the shaft can be used to lever all of the plates back up at the same time. Put a wheel, or a lever, on one side of the shaft with a rope or chain going back to the firing line. Remember a counter-weight so the shaft/cams rotate down out of way after resetting the plates.