I'll share a story...
CTbuilder and I bought some KCI mags specifically to pull the guts out of, fill with lead, and duracoat blue to use as weighted (inert) training mags. When I brought them to his house we were on the way to the range and somehow two of the KCI mags made their way into the range bag. How do I know? Because the only two equipment-induced malfunctions that I've ever experienced with a Glock firearm occurred that day.
Buy OEM Glock mags.