The 223 runs lower muzzle pressures than I'm used to seeing, due to having a smaller case capacity than anything I use. Even my .204 Ruger had about 30% more capacity.
Anyhow, a 16" barrel with 25.0gr H335 under 55gr bullets is going to run just shy of 11,000psi, while a 20" barrel would run 8,500psi.
8,500 is close to what a "normal" rifle would run, like a .308 or .243 with a 24" barrel.
11,000 is going to be loud. My 15" barrel Encore chambered in 7mm-08 runs about 14,000 and it's much, muUCH louder than a typical rifle.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.