i like the round. the abundance and the knock down power the 7.62x39 have. also i have a bit of a stock pile of 4k rounds that i am just spending with my ak. I have been curious about the piston platforms on the ARs but i would have to do research because i am not too familiar.
I have been looking at the AR308's pretty nasty I would get one if i had that kind of cash
Personally the ballistics of the x39 are horrid and you cant utilize the AR for what its made for with that round... I only went x39 to keep my AK a true AK for the most part.. And there is no such thing a knock down power just FYI.. the only thing one bullet has over another is penetration of materials and tissue damage.. There is no blunt force great enough for an actual "push" from a bullet even 12g slug.. It goes through you and just hurts like hell.
that being said if your set on a AR platform the calibers id recommend HIGHLY and probably only calibers id use to utilize for that platform (yea i get deep into this guns were an obsession for a good while) is the 6.5 Grendel, ***300 black out**** or the standard 5.56.
The black out is a 30 cal round like the x39 but with way better ballistics and its pretty hot right now Daniel defense makes great 300 BLK uppers..
the Grendel is more of a target shooting round with great velocity and ballistics more popular with target bench shooters.
the 5.56 is my caliber of choice because its the norm has decent ballistics and good tissue damage and it fragments in the body and walls.. also has a good following of different ammo companies and types of ammo for different uses.
That being said since you have a crap ton of x39 its understandable why you want to go with that.. But I say use it as an excuse to get the better calibers and let the Ar platform shine and have an excuse to buy more ammo
But if your dead set on x39 im not sure which companies make x39 uppers most probably will or specila order one.. but only companies i trust for AR platforms are daniel defense, BCM** (personal fav cheap and rugged and very well made), stag, spikes, LMT, La rou, LWCR (slightly pricey but nice rifles), Noveske (again slightly pricey but very clean well made and the name gets brownie points with the gun guys), OAK armorment, Rock river (slightly over priced for what you get IMO), smith and wesson,
pretty much in that order..
Hope that helps brotha.. Like I said I kinda of had an obsession with my firearms for a while and have put tons of net/book time to know whats on the up and up when buying and researching.