Most companies (including Shooters Design) make there metal slides out of milled solid piece 7075 Aluminum. As I understand it, it is a very high quality and light aluminum alloy. Common uses seem to be golf clubs, airplane parts, and well pretty much any thing that needs to be strong and light. Plenty strong enough for a GBB and light enough to cycle properly.
Definitely not "pot metal" though I am sure some companies may, and probably do use much cheaper metals, so buyers should do there research I guess.