I have a Custom Colt 1911 Light Weight Commando and I doubt if I ever see it wear out regarding the frame. I like the steel frame guns myself. But regarding your question about alloy frames. It brings to mind all the rounds and abuse the Beretta M-9 Military Pistols take. Never seems to be a significant problem with the Alloy Frame, at least I have never heard of it. So I doubt if you and I would ever see it no matter how many rounds we shoot. And I know some (Frames) of them had more than 30,000 + rounds through them on the low side. So i guess who knows how many rounds it would take to wear one out. Also would obvioulsy depend on the loads and other effects.
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