Steel and perhaps a 30 max. I have 20's and 30's. The 40 round has capacity, yes, but damn can those things get in the way of quick movement. And I like steel because I can buy a 40 year old steel magazine that's still perfectly useful. Perhaps with some of the bakelite mags that can be true too, but modern plastic mags still need to pass the test of time in my opinion.Wondering what you would pick for a SHTF or combat situation and why? also would you choose steel or plastic for the magazine?