I wanted to convert a CAR-15 to 9mm, but didn't feel like sending all my UZI mags out to have the extra cut added. Yes, I'm a cheapskate.
Anyway, I stumbled upon the VM Hy-Tech 9mm UZI Mag Conversion Block by Ameetec Arms. This adapter is longer than the other AR/UZI mag adapter blocks on the market, and has its own latch which engages the existing slot on an UZI mag. At $145, it isn't much more expensive than other mag adapter blocks, and of course saves the time and expense of adding another slot to each mag. The adapter is held in place by the existing AR mag catch, so it's easy to install/remove.
There are a couple of caveats. While the older military mags fit well, the newer 25 and 32 round mags are too tight. I spent about an hour with a file and emory cloth to get it to accept all of my mags. It also lacks the ability to hold the bolt open when the mag is empty, but this is a minor issue to me.
Today was my first opportunity to try it out. Keep in mind that this was a semi-auto test, but it functioned perfectly. I loaded ten 25 round mags before heading out, and emptied every one without a single stoppage of any type. I tried it with mags that fit snugly, and mags that were loose. Every one fed fine.
I can recommend this product without reservation.