I built my first 9mm AR years ago, when Colt and Olympic were competing with their non-compatible systems: the Colts (& clones) used Colt subgun mags, and (IIRC) the Olys used Sten mags, with some type of magwell adapter attached to either the mags or the AR. Also available was a third option: a magwell adapter from a little company called VM Hy Tech, that let you use unmodified Uzi mags with a Colt-system upper. (Pretty neat!) For that first 9mm AR, I
almost went with the VM Hy Tech adapter and Uzi mags, but finally decided on one of the new CMMG dedicated lowers and Colt-type mags instead.
Unfortunately, VM Hy Tech went out of business shortly after that, and I instantly regretted not buying one of their Uzi mag adapters (which are just about 'unobtanium' now). So, I built my own magwell adapter, using a surplus military Uzi magwell, combined with the front & rear portions of a Colt-type magwell adapter (which provided the feed ramp & ejector). The Uzi magwell is too wide to fit in a milspec AR15 magwell, so I used a Cavalry Arms Cav15 MKII lower, which has a magwell wide enough for .45 "grease gun" mags (or an Uzi magwell ;^). The rest of the carbine is mostly standard Colt-system parts. End result, a lightweight 9mm AR "fun gun" that uses unmodified Uzi mags (one of my favorites)!
As always FWIW, IMHO, YMMV, etc., etc.
