Re: .223/5.56 62gr 3000fps in 16" Barrel
Those velocities are more safely found with a 20" barrel. There are 2 approaches to reach what you need/want.
1) Keep the current optics and switch to a longer barrel to safely reach those MV's
2) Keep the current rifle configuration, figure out a load that shoots nice, tidy groups, and put a different optic on there.
I'd personally pick the second option if you have a rifle that already shoots very well. The glass you have will get you a better return on investment than selling off a used barrel and putting a new one on there.