Time to stop shooting at steel plate

OK it's been fun up to now. No harm done but after turning 1/8 then 3/16 inch steel plate into Swiss cheese with Chinese 123 grain strel FMJ over steel core shells I think we'll back off the quarter inch plate in case we start to get unwanted deflections. We knew for a fact that it would go through the 3/16 but no sense second-guessing something thicker. Question is, now what to test the penetration of this stuff on? I'm thinking sandbags. many in a row because I also want to compare it to my 170 grain 30-30 soft points