There are tons of piston AR-15’s out there, many having been produced for some time. And most of them are 5.56 . There is no reason to go to 6.8 except for novelty. As others have stated it’s not significantly better at longer ranges than 5.56- the initial impetus for this program.
CC
As Fai Mao mentioned in Post 33, the caliber change may be due to the need to penetrate body armor.
From 2018:
https://uklandpower.com/2018/10/31/the-us-army-special-operations-command-usasoc-6-5-mm-precision-intermediate-calibre/
"The 6.8 mm ammunition around which future weapons are to be designed are likely to generate much higher chamber pressures “to ensure that the rounds can still blast through enhanced enemy body armor at up to 600 metres.” Such performance is expected to be achieved by using pressures similar to those of a tank gun, i.e. up to 550 MPa (80,000 psi), instead of usual 5.56 mm pressures of 380 MPa (55,000 psi) for the M855 and 420 MPa (60,000 psi) for the M855A1."
The quote in the article says 6.8mm but the link and the rest of the article cites 6.5mm.