Hi,
In the book "Stopping Power: A Practical Analysis of the Latest Handgun Ammunition" written by Evan P. Marshall and Edwin J. Sanow, there is a whole chapter devoted to the Federal EFMJ round. It was written by the designer of the round, Tom Burczynski who also happens to be the designer of the Hydra-Shok rounds. In the chapter, he describes the thought process, design, testing and production that went into the manufacture of this new round. Penetration was a major concern during testing. I quote:
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To complicate matters, an opposing problem existed-penetration needed to be increased. What at first seemed doable was now looking downright impossible. We needed a bullet that was weak enough to expand in relatively light resistance and one that would remain intact when fired through steel while providing adequate penetration. I was beginning to think that unacceptable terminal ballistic compromise might provide the only solution."
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He further describes the search for the right type of bullet to make EFMJ to work. I quote:
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The stronger jacket and heavier bullet had gained us some needed penetration. None of the jacket straps snapped in two, and the weight retention was 100%. But the real test of bullet robustness was yet to come-hard barriers."
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Here lies the answer to your question concerning enough penetration. He talks about the many calibers they tested and how penetration still was not acceptable. They went through many tests but what was determined was that hard barrier tests i.e. 20 gauge steel, windshield glass, 3/4" plywood, and wall board, showed that a thicker jacket was needed. They were having problems with the bullet jacket snapping. In the end, they strengthen the jacket and increased the bullet weight. To make a long story short, I will quote the end summation of the chapter.
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In summation, the EFMJ concept provides a number of advantages. It looks and feeds like hardball and expands to a maximum diameter faster than a hollowpoint under all conditions. Unlike the hollowpoint, its main advantage is that it never clogs up and therefore invariably expands when fired through dry media like heavy clothing, wallboard, and plywood, and provides tactical penetration against hard, intermediate barriers."
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The book showed that a 230 gr .45ACP EFMJ round had a penetration of 14"+, however the chamber pressure had increased to the +P range. In the end, they set the bullet of the .45 ACP to 200 gr. The 9mm and .40 is also available. I chose the .45 ACP as the focus since this is what I shoot. Unfortunately, the round is still too new so there is no data in the book concerning stoppage vs.failures to stop with this round. Here is a link to an article written about a year ago concerning this rd.
www.ammoman.com/40_EP_DESC.htm