You don’t mention bullet weight...200? 250? 300? 340? Cast or jacketed?
My Trapper has a 16.5” barrel and +P loads give fine velocity. No problem getting 250/255s up to 1700 fps or 300s to 1500 or 340s to over 1400. But I find that I don’t really need those speeds, dropping them by 200 fps gives similar terminal performance and trajectory with less recoil, blast and stress on the action. Ballistics by the Inch doesn’t show a great difference between 16” and 18”, certainly not enough to matter in the real world. If you are obsessed by the last few fps then the 18” barrel will give higher velocities. Depending on the load expect at most 50 fps more velocity depending on the bullet weight and the powder used.
http://www.ballisticsbytheinch.com/45colt.html.
