Well we know 5.7x28mm does nothing to this plate, and the solid brass .338 just left a big mark in it, so how about tried and true .30-06 M2 AP rounds :) Circa 1954 (per headstamp).
Gun of choice for this test is a Winchester Model 70. 1965-1967ish build. Redfield 2-3/4 power quick detach scope.
Two shots hit the plate, with the second knocking the plate from the wood. We fired a third shot at a stainless steel steam trap (not pictured), and it had no problems punching through both sides of 1/8" thick 304 stainless steel.
I think next we'll try M855, and if I can get my hands on one, .50BMG :)