Blueduck...a search will definitely turn out some good information on 9mm ARs. AR15.com also has a forum dedicated to pistol caliber lowers.
However, I'll try to answer some of your questions here..
Will "Hi-caps" be available?
The 9mm ARs typically in two categories. Colt style that use Colt and modified UZI magazines and Olympic Arms style that uses Sten magazines. There are some Colt type that are also modified to use STEN magazines as well. Most folks that I know use the Colt system with a mag block.
Sten magazines will always be around as well as UZI magazines. Modification to the UZI magazines for the Colt system consists of just cutting a mag release cut in the mag body. However, UZI mags are no where near as reliable as the Colt magazines. Colt magazines are still available, but at a premium. You are looking at close to $100 for a magazine. I don't anticipate this to change as Colt became politically correct with their ARs before they were required to. I doubt they will produce any Colt 9mm mags for the general public use. The one ray of hope will be that LEO mags will no longer be such after the ban sunsets. That should put a few more on the market.
Can a .223 upper be used with a 9mm lower?
If you have a dedicated Colt 9mm Lower with a pinned in mag block, No. If you have a standard .223 lower and use a removable mag block..then yes. Installation and removal is as simple as turning an allen wrench until it locks into place.
What manufacturers have the best reps with these models?
Colt and Olympic Arms and Rock River Arms are the three I know of best. Rock River uses the Colt System.
Same direct impingment system as AR?- more or less reliable than the 5.56 or .223??
With Colt or Sten magazines that are just as reliable as the .223. The system is a blowback design. There are no locking lugs on the bolt and no gas tube. The downfall is that it leaves a lot of spent powder residue and unburnt flakes in your trigger group. It's not a serious problem, but you will need to clean it out ever so often. And whatever you do, do not use Carb cleaner to clean the trigger group. The last time I did this the cleaner crystallized the flakes onto the sides of the reciever and trigger group. It took me a good while to get it all out.
Artherd...
The 9mm comes out of a 16" bbl signifigantly hotter than a pistol,
Not really. The 9mm comes out approximately 100-300 FPS faster depending on the load and weapon used for comparison. Here are my findings from chronographing different loads.
Remington 9mm bulk pack 115gr JHP.
Kahr T9 4.0" barrel= 1146.6 FPS
9mm AR15 14.5" barrel= 1305 FPS
load #1 Winchester USA 115 Gr FMJ ( White Box 100rd count Value Pack)
Kahr PM9 3.0" barrel : 1020 FPS
AR15: 1280 FPS
Load #2 Speer 124 Gr +P Gold Dot
Kahr PM9 3.0" barrel : 1093.4 FPS
AR15: 1374 FPS
Load #3 Winchester USA 115 Grain JHP (White Box 50ct)
Kahr PM9 3.0" barrel : 989.5 FPS
AR15: 1212.2 FPS
Load #4 Winchester USA 147 Grain JHP (White Box 50ct)
Kahr PM9 3.0" barrel: 863.82 FPS
AR15: 1002 FPS
Load #5 Federal Personal Defense 135gr JHP
Kahr PM9 3.0" barrel: 926.64 FPS
AR15: 1133.5
None of these loads are "screamers". If I did use a 9mm for HD I would look at the Corbon 115gr JHP. I don't have any on hand to chronograph, but I'd suspect they would go about 1500-1600 fps out of an AR15. That should give some pretty good performance out of a 9mm with minimal if any overpenetration. Want a real screamer?..try Corbon powerball ammo 100gr . 1475 fps out of a handgun. Should do very nicely in a 9mm AR.
Good Shooting
Red
Bedside deal is fairly minor factor anyway, just don't really like to own any guns strictly as play toys.
