Originally Posted by cetane
The metal ones are plastic inside too, just a metal shell. I've got some CA metal mids, and mag plastic mids. Both work fine. I've got some other plastic mids that don't feed anymore but I think the springs are shot (or they are completely full of dirt :-/)
+1 to the above.
If you tend to drop your mags, or be overall rough with them, get a midcap with a metal shell. Otherwise, plastic ones will give you a better bang for your buck.