Dr. Ashby's research show that aluminum inserts are a potentially VERY weak point in a screw-in setup. As always, Ashby is not looking into "best case" scenarios with no bone impact, or light bones in small "big" game. He is advising us on how best to set up our arrow systems so that they will penetrate and kill even with the worst-case hits into heavy bone. In the previous issue of TBM, if I recall correctly, he says flat-out never to use aluminum inserts. Steel are preferably and brass generally OK unless you're going for elk or bigger. Good of you to care enough about the animals we hunt to ask!