In my opinion your desire for a short rifle should take precedence over a predetermined velocity. The rifle that works for you and puts your bullets where they belong will get the job done. That said, my Marlin 45-70 has an 18.5" barrel and I easily get 2000 fps with 400 gr bullets. With the 416 Rigby I suspect you can get all the speed you want with the same length.

Take care, Tom