I have.....a lot. I started buying mags back about 1985 and whenever I found them cheap, I bought all I could as investments. I still buy them when they're cheap.
Last year (early last year
) I found a surplus dealer who wanted to get out of the mag business, and bought a very large carton nearly full of GI 30 round mags for $3 each. I figured they wouldn't go down in value and they haven't. I'd buy more if I could find them cheap again, but that's not looking too likely to happen.