A friend I work with just purchased a Colt AR 15 chambered in 7.62x39. The question he had was what are the differences between the 5.56 and 7.62 versions. Are the lowers the same? Could you replace the 7.62 upper with a 5.56 upper? Ate the bolt carrier assy different?     (knowing the bolt must be).Any and all info will be greatly appreciated.

ASO544

The lowers are the same and you can interchange the uppers but the bolts are different because of the size of the case.
just my 0.02 Ron

The only differences between the 5.56 & 7.62 versions are in the the barrel and bolt head.  The lowers are identical, and the bolt carrier is too. If your friend wants to, he can buy or build himself a 5.56 top end and have two for the price of one and a half.

The 7.62 unit I had some years ago was the most accurate, by far, 7.62x39 rifle I've ever owned. The only other gun I've ever had in this caliber that even came close was a Valmet M62.
The problem with the gun, and the reason I don't still own one was the magazine capacity  Colt used a standard 20 round M16/AR15 mag blocked to 5 rounds, and it worked OK. However, using unblocked mags or 30 rounders didn't work for more than 5 or 6 rounds because the much greater body taper of the 7.62 round requires a bannana shape mag like the AK uses.  Several companies brought out 20 or 30 round mags that were straight where they fit into the action, and curved underneath like the AK.  I tried several, and couldn't find a brand that worked reliably.  Now that was ten years or so ago, and someone may have licked the problem by now.  I'd encourage him to pick up one sample of any extended type mag he can find and give it a try.  If he finds one that will give him 100% feeding three times straight, he should get some more of that brand.

I have a Colt 7.62x39 upper, and I was amazed at how well it shot.  With the old Hornady .311" 123gr soft point bullets it shoots 5 shots under 1" at 100 yards.  It runs fine in full auto as well, using the magazines that have a slab side feed tower with an AK bottem TiG welded to it.  There are four different versions of these, and only one actually works without mods to the follower to get it to sweep cleanly past the feed tower junction.