March 17th, 2017, 05:26 AM
Join Date: Feb 2014
Location: South Texas
Any AR15 lower. Can even swap the 5.56 & 6.8 uppers on the same lower. I did that on my first 6.8 upper until I got around to building another lower.
Only issue might be the buffer weight or spring (or gas port) may need tuning. Depends on the gas port sizing on the barrels. Some barrel manufacturers tend to oversize the gas port to handle light to heavy loads and then shooters find themselves putting in heavier springs and heavier buffers. Some 6.8 barrels try to avoid this and size port for specific range of ammo. You may need to fine tune the port size (drill little bit more) or use an adjustable gas block if oversized.
Have built 3 6.8s and have not had to fine tune using standard buffer and spring.