Can you do this??

Yes indeedy. If you want to go 1st cabin, put a Barrett 6.8 piston upper on a good lower and use Barrett magazines. Don't know why, but we had a little problem with SSA 6.8 with the 110gr Accubonds. The Hornady factory ammo with the V-max bullet performed great. I'm also waiting for LMT to do a 6.8 quick change unit for their 5.56 piston upper.

Any ar-15 lower will work. Need new mags...I think??? Someone was saying you can use 5.56 mags but only to the tune of 5-10 rounds per mag. I don't know if that is true. I have 2 Barrett mags and use those with my 6.8 upper. I have had good luck with all the ammo I have tried. I hunt with the 110gr TSX from SSA.

Perry

I have used 223 mags for the 6.8spc,they work.
You just have to reduce the number of carts in the mag,something like 18 in a 20 and 25 in a 30.

If you are seriously interested in the 6.8 you need to check out the 68Forums.

A 6.8 upper can certainly be run on an standard AR-15 lower. Mine works just fine.

Different mags are required as the ridge designed to help stack the rounds on each side of the magazines near the front cannot be as deep for the 6.8 as they are on a 5.56 magazine. Plastic mags are not offered in the 6.8 as they would swell when fully loaded and not fit into the magwell (so says MagPul).

Be careful as you look for an upper. The good setup these days is a 1:11 twist, with 4 grooves (though WOA gets away with 6) and an SPC II chamber. Older rifles with 1:9 or 1:10 and the standard SPC camber will not make the higher velocities achieved today ... and should not be used with the "tactical" loads provided by SSA.

The 68Forums really is a good place to frequent if you are interested in the 6.8!

I would just add to the 6.8 twist debate that if you were looking at LWRC you should ask them about their uppers and tactical loads before ruling them out based on twist rate. For other manufacturers Mike's post is dead on. Of course there is also the DMR chamber in addition to the SPC II.