does anyone make one? (barrel)

Not that I can find.

DPMS does make 20" 7.62x39 stuff, but you already knew that right.

[url]http://www.dpmsinc.com/[/url]

[edited to add] Yes actually. Model 1 sales has them. [url]http://www.model1sales.com/item-detail.cfm?ID=BCE167&storeid=1&image=bar16762.gif[/url]

To get an operational 7.62X39, do you just replace the upper?  And what about mags?

Upper plus BOLT. You can use any GI mag with 762x39 HOWEVER. you will only be able to shoot 5-6 rounds per mag.

The mags are the big trick you have your choice of USA (dont laugh they work!)
Franken Mags (I dont know where they are built or the real name) it is a AK mag with extension welded in place.
Colt 762x39 mags are colt 20rndrs that hold 6 rounds....

Use only A2 butt stock!
The colapsible stocks make the 762 screw up....Dont try and waste your time they do i tried I wasted my time. Something about the bolt doesnt have enough momentum to carry the next round.. I really dont know but Colt made them with 16 inch brls and A2 butt stocks and now I know why.

Here's about as close to a 7.62x39 M4 as I've come:

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=15354[/img]

The only other modification I would consider is to maybe have to barrel trimmed back to 14.5-15" and have a flash suppressor permanently attached.

I don't think you are going to find a standard profile 7.62x39 barrel with the M4 style barrel cut as this would weaken the barrel too much.

The only differences of the 7.62x39 AR are in the barrel, bolt, ejector, and in the case of Colt, the unmarked A2 elevation wheel. For more on magazines, see my post in [url=http://www.ar15.com/forums/topic.html?b=3&f=17&t=165842&w=searchPop]7.62x39 AR-15 mags[/url].

Unlike 1ar4me, I've had a great experience with the M4 4 position collapsible stock on my 7.62x39 AR15. I also use the Colt "H" marked heavier carbine buffer. Other members have reported similarly favorable experience using collapsible stocks on their 7.62x39 AR15s.

[B]M4K[/B] • Model 1 Sales 11.5" - 7.62x39  w/Colt 4 pos. CAR stock
[img]http://www.z06vette.com/gallery/data/500/13511_5_5_3a03.jpg[/img]
Runs like a steam locomotive in FA.  - Western Metal 20 rnd mag