I was wondering if anyone here has any experience with an upper in this caliber and if so, would you recommend it to anyone.  I like shooting this cartridge in my pistol and thought it would make a pretty cool pistol caliber AR.  I am considering this vs. a .50 Beowulf, or 6.8 SPC (I know, apples and oranges) so I would appreciate any feedback.  Thanks in advance.

+1; been thinking about this myself.

Kurt Wala of Kurts Kustom Firearms built one for GW/LE magazine, you should inquire about it in his forum.

I built a .357 sig upper in the Colt type system and it works great -- just the ammo costs a bit much for doing any blasting. I had kurt do the barrel and open up the bolt face on a 9mm bolt. I started out using my sten mag system but later switched to using uzi mags because the stens were a pain  loading those fat bottlenecked 357sig cartridges. The Uzi mags are a dream to load but you can only stuff 3/4 the amount of rounds in them due to the size of the cartridge compared to a 9mm. There are a few things that are needed to make the 357 sig work -- an extra heavy buffer, a heavy Wolff buffer spring. I bought the "X" buffer from SAW Sales and it weighs about 8 oz.
The best ammo I've shot has been the Federal 125gr Premium JHP and Seller & Bellot 140gr FMJ. Believe it not the Seller & Bellot is the most pleasent to shoot. Reliability of the rifle depends mostly on the feeding system you use and the magazines. You could get Kurt to build and test the upper for you -- he may build in either the Colt style system or the Oly system now.

AR Fan - Thanks for the info.  I have been emailing back and forth with Kurt and he seems very knowledgeable and has been very helpful.  Would you mind sharing some of the specs of your rifle - barrel length, contour, upper and lower that you are using, etc.?  I agree on the ammo cost being kind of high in that caliber.  I have also found that in my pistol the S&B tends to be the most mild load.  Not sure if they are just not loaded as "hot" as some others or what it is.  As far as the magazines go, do you have any problems with misfeeds?  That is my biggest concern with the system is the reliability of the magazines.

I love to shoot the Seller&Bellot ammo -- I just wish I could find it in HP's like the federal with the hydo-shoks.

The barrel is a standard carbine profile -- light under the hangards and 16" long. The Sten mags are the best for reliability but due to the diameter of the cartridge it is hard to load more than 18 to 20 rounds in a mag. The Stens will need to be adjusted to work right. The Uzi mags work good too and can be used with a Colt style magwell block. The Uzi's need to be adjusted also but they are the best for at the range or plinking (a little expensive plinking) since they load fast and easy. Joe Black at www.m60joe.com/ does magcatch slots in the Uzi mags and can add a bolt-hold-open tab to the Uzi's. Don't even think about Colt 9mm mags -- they do not like the .357 sig ammo and they cost a lot so you will save some money there.

the magazines are the biggest pain-in-the-neck part of making the system work and you with have to be ready to do a lot of work on them to get them just right. Kurt used Uzi mags that were double-stack, single feed and originally made for the .40 S&W round which is the same diameter as the .357 sig. The problem with those mags is the price -- and being able to find them. You might want to think about just getting Kurt to set the whole thing up for you -- it might seem expensive but in the end you will be saving both time and money.