Lots depend on the round fired though, stopping a regular fmj vs stopping something like a Raufoss Mk211 or SLAP round is a world apart!
Just don’t forget to remove the muzzle break if you intend to use SLAP rounds or things could go wrong!
As for backface deformation, armour is usually tested with a clay backing to capture the imprint.
Last one I wore was rated to .308AP, ceramic with some metal backing from memory. 10m test shots didn’t even dent the back.