The obvious theory of why many reputable companies (S&W, Ruger, GSG, SIG, Colt etc...) have elected to use Zamak (or an equivalent thereof) on their respective slides is 1) it cuts down quite a bit on costs and 2) the vendors belief that the .22lr does not call for an all-steel/alloy/aluminum slide compared to its big sister the .45acp.
As I said earlier, my GSG has been rock-solid after a couple of K downrange--yet if I was not evaluating my .22lr candidates with money being a key variable in that equation, I jump on the RIA or CZ (not too familiar with the others like Kimber in .22lr). If one is willing to spend $500+ on a .22lr range plinker it's a done-deal.
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“A free people ought...to be armed..."