Check this out, Doublestar is making a 9mm dedicated lower for the AR family.  And it looks pretty sweet!

I apologize as I don't have a way to post the pics directly. 
http://www.shop.doublediamondsupply.com/AR-15-Lower-Dedicated-9mm_c7.htm

and pics of the assembled rifle
http://sigforum.com/eve/forums/a/tpc/f/630601935/m/233107032

Could be a lot of fun for a plinking/trainer.

ooohhh, now if they made it in 10mm or .45 I'd need a new keyboard because of the drool.

The best I can think of for big 10mm magazines (besides MP5-10 mags that I doubt are available anywhere) would be modifying the feed lips on M3 mags.

I was just figuring that since high capacity 10mm basically don't exist, and the only existing magazines long enough to fit 10mm are for .45acp; it would make sense to go with modifying the cheapest and most readily available .45 hi-cap mags - M3 Greasegun mags.

It looks like I'm far from the first to think of it:

Interesting Glock doesn't make a 10mm magazine larger than 15 rounds.  http://www.glock.com/english/index_magazines.htm

Geoff
Who notes the factory Glock 33 rd mags make a Kel-tec 2000 attractive..http://www.glock.com/english/index_magazines.htm