Welcome to the site.
You can build the 6.8 to any length you want, you will still gain performance, not a ton, but it isnt like the bullet starts slowing down with barrels over 16 inches in length.
I have an 18 inch upper that I really like and I have a 16 that I like as well, different uppers for different tasks.
You want the SPCII with a 1-11 or 1-12 twist barrel the 1-10 doesnt give you any thing and increases your overall pressure.
Feel free to ask away.
You can build the 6.8 to any length you want, you will still gain performance, not a ton, but it isnt like the bullet starts slowing down with barrels over 16 inches in length.
I have an 18 inch upper that I really like and I have a 16 that I like as well, different uppers for different tasks.
You want the SPCII with a 1-11 or 1-12 twist barrel the 1-10 doesnt give you any thing and increases your overall pressure.
Feel free to ask away.