drf,
Here's the skinny.
Sept 13 1994 brought the "Assault Weapons Ban" which limited a bunch of silly things for sale in the market place.
One of the stipulations was that the manufacture of magazines that held more than 10 rounds (unless only sold to Law Enforcement or Military) was illegal. Ownership is not illegal. Now, ownership of something designated LE only or Military only might be illegal. I'm not sure and I'm not going there personally.
Providing Congress, the Senate, and the President don't all get together and renew the AWB on Sept 14, 2004 (I beleive just after midnight of Sept 13) manufacture and sale of normal capacity magazines (what some call hi-cap) resumes again. Blood will not flow forth into the streets and logic will prevail. We hope.
It's still perfectly legal to go out and buy overpriced magazines that hold more than 10 rounds though. Now, howh do you determine if it was made before 1994? Well, you pretty much can't. If it says "Law Enforcement Only" on it -- don't touch it. That's shakey ground and might just be entirely illegal. I don't know.
It's also prefectly legal take a magazine for one gun and shove it into another gun -- provided that it still works in the original gun! You can modify a Beretta 92F 15 round magazine to work in a Springfield XD9 I hear. So long as it still works in a Beretta that's legal. You can take a stock Springfield XD40 magazine and load 15 9mm rounds into it and shove it into the XD9 too with no modifications. Still prefectly legal.
If you own pre-AWB magazines and they are damaged you are permitted to buy repacement parts for them also. How you prove you had a pre-1994 magazine, I don't know; but, according to information from the ATF that's been relayed onto here, it's legal.
Oddly enough, Dianne Feinstein's pet "evil guns" the AK-47 and Uzi probably have the cheapest 30 round magazines on the market because there's millions of them floating around.