Curiosity as well as availability and lower price got me wondering...

I spoke with Customer Service from Feather USA, the makers of the AT9 and RAV-9 quite some time ago when I was considering all my 9mm rifle options.  I was told that the RAV-9 will accept Uzi mags with a notch cut into them...

SO...

Wouldn't it stand to reason that, since Colt-style AR's can use converted Uzi mags, the Feather mags could be notched to fit in a RRA/Colt AR?

Why do I even bother to wonder?

Because CDNN has Feather 25 round mags for $19.99, and Uzi 25 round mags for $24.99.

My 9mm AR project isn't off the ground just yet, or I'd give it a shot myself...

Get $11 25 rd. UZI mags from DSA.  I did.  My RRA block is now sporting grooves in the feed ramp and my UZI mag seems to feed much more smoothly.

I don't know anything about the origins of the Feather AT9 mags.  I shot a Feather carbine back in the 80s before I knew what was junk and what wasn't.  If the actual gun is any indication, the mag should be avoided. YMMV.

If you're going to go with a Colt pattern setup, I can say that the RRA upper I got from Pete-in-NH is pretty nice.  I dig the slick side upper.  It's not cheesy like the ASA upper with the ground down forward assist lug.  The mag block is great if you're going to use Colt mags.  Be sure to tweak the ejector all the way in toward the bolt.
If you're going with just UZI mags then another block might be a better choice.  There are drop-in-the-top adapters that apparently work with the converted UZI mags 100%.

Really?  Thanks for the tip!

Who makes 'em, and where can I get one?

I do plan to go with Uzi 25 round mags for the sheer cost.  I will be getting a RRA upper over the Oly, for the left-handed friendliness of the big fat "clamshell" deflector.

I'll be ordering my upper in the next 4 - 6 weeks... Can't wait.  (but I'll have to, since there's another 4 - 6 week wait on delivery!)

ASA (gag!) makes one.  Look for adapters on the Brownell's web site.  

Don't go with a Colt just because of the deflector.  It's $12 and fits on an Oly upper.

Sten mags are cheap and hold more but UZI 25 rd mags are still pretty cheap and are way easier to load without a tool.

UZI mags can be made to work with the RRA block.  LRM will convert them for a pretty fair price and does first rate work.

You might want to post a topic about drop-in-the-top adapters and where to get them.

Hmmmm.  How long ago did you get the DSA mags?  I've checked out their site, but I haven't seen any cheap mags.  Maybe I'm just looking in the wrong place.

DSA used to have 25 rounders for $14.95 new, or $10.95 used.  They've been outta the used for a while, and it seems they're outta the new ones as well.  

I'm pretty sure they were there less than a month ago, not that it matters now.

Sorry about the bum steer.