If you just need to swap go at least 20”. But honestly I would run what you have. I wanted a lighter 308 ar build for hunting to run suppressed torn between 16 or 20, ran the numbers and a 20” only gains me approximately 50 yards more before going transonic over a 16” barrel. Guess which way I went on this build, 16”.
FYI, my 20” only shoots 168’s at 2550 fps.
Nice rifle btw, that’s definitely from when Mega was making GGP’s sets. Solid