sorry for the dumb question but just ordered a .308 lower and am hoping I can shoot both x51 and x39 rounds?  If so what mods are needed to accomodate each round?   Snce the x39 is so much cheaper, am hoping this is possible.  Thanks for the answer anyone.

I'm pretty sure that 7.62x39 uppers and magazines are made to fit standard AR lowers (5.56/.223 lowers).

Please step back from the workbench and learn about what you are doing before you proceed.  These rounds have hardly anything in common other that bore size of the bullet.  





These are completely different uppoers and lowers that handle this round.  As the poster earlier said, an AR15 5.56 upper can be modified by changing the barrel and bolt to shoot the 7.62x39.  





If you want a cheaper gun to shoot go get an AK.  If you want a rifle that shoots big boy 7.62x51 rounds go with that, only option is to find some surplus military rounds.









 

Is it possible.....sure, IF you have a machine shop and the ability to make all the parts you need.  

But, with using what is out in the market now...NO.
A normal 7.62x39 (AR15) upper will not fit on a .308 (AR10) lower.  I guess somebody could make a bolt carrier in the AR10 size that worked with a 7.62x39 bolt.  You would also need to make a magazine that one hell of a large block to take up the extra room of a 308 size magazine.  The investment to design it and make it all work would be multiple times more than the cost of a 7.62x39 AR15