Heavyarty,
You are right. Original AR-10 designed by Eugene Stoner
(and still is manufactured by Armalite) is chambered in .308
Winchester/ 7.62mm Nato (7.62X51). Later on, they changed to .223
Remingon/5.56mm Nato and desingated AR-15 / M16.
There also is one that takes 7.62X39 (russian) as well. I don't
own it, but has seen them floating around. Here is the link for
it.
http://www.gunblast.com/Doublestar-762x39.htm and http://www.gunsamerica.com/guns/976532802.htm
Prior to Clinton's 94 Assault weapon ban, it was readily available,
manufactured by Olympic Arms, Armalite/Eagle and Colt. They
didn't call it AR-10, but if I remember correctly, they referred it
just as AR-15 chambered for 7.62X39 (and each manufacturer probably had
different designations). Once in a while, I see them advertised
in Shotgun News, but I haven't really had any need for it. They
also sell conversion kit, which is just an upper receiver barreled for
7.62x39. Cost of it is about $450 +. You can get a complete
AK-47 at around $350. There also was some reliability issue with AR's
that were chambered for 7.62x39mm, so there really was no advantage for
owning AR in that caliber.
This is why I love Ar-15 over other rifles. You can get
conversion kits to change to other calibers such as 6.5mm, .50, 9mm,
.45ACP, 22LR and so on and on.
Sometimes, I want it all, but wife won't allow it, so for now I only have in .223Remingon/5.56mm Nato and .22LR.