Re: 6.8spc
6.8 isn't a long range cartridge by any stretch, you are looking at 300-350yd max for clean kills on medium sized game. Makes a nice lightweight hunting setup though, recoil is very mild...bullets from 85-130gr give you some options on bullet weight and speed. The lighter bullets in the <100gr side can hit up to 3000fps, the heavier 130gr can still get up to 2500fps with the right barrel specs.
If you are planning to build a 6.8mm as a hunting rifle from the ground up, an 18" midlength is almost ideal. If you are just building a 'typical' home defense/shtf carbine 16" is fine, and the 115gr OTM loads by SSA will be more than adequate for that role.