Yeah but it's not because they're made by Echo 1, that has nothing to do with it. Infact, I'm almost positive that the E1, G&G, TM, and CA G3s can use the same magazines. I don't know of a single Echo1 gun that has a proprietary magazine, any one of their guns that is the same "style" as another gun by another decent brand will use the same magazines.
Edit: I guess I'm kind of reading it wrong still but, uncommon magazines has nothing to do with the brand, really, at all.