I did a bunch of tests some years back, over my Chrony with GI 55gr ball ammo in 20 HBAR, 16 CAR, a HK 53, 14 T/C and several bolt actions out to 26". Bottom line, the velocity change was 29fps/inch of length change. So, you should lose around 120fps.
All barrels are different, but 30-40 FPS/inch is probably very realistic. And it will vary with bullet velocity as well as with each individual barrel.
I find the 16" barel handier to use and I don't worry about a little velocity loss in the 223 Rem AR. I doubt I'll be shooting a 223 Rem AR over 300 yards regularly anyway, so any ballistic difference is going to be nil for the most part.....if you can see it at all.
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