Noel,
The Hornady Labs list an approximate variation in bullet velocity based on their experimentation. For bullets traveling in the range of 2000-4000 fps, the velocity will vary from 10-40 fps per inch of barrel, respectively (Hornady Handbook, 3rd Edition, page 401).
The Springfield Armory tests showed the 30'06 ball to range from 2709 fps in a 24" bbl to 2848 in a 32" bbl. Another Springfield Armory test indicated an *average* (5-shots) drop of 83 fps when shortening the bbl from 26" to 24" (Hatcher's Notebook, 3rd Edition, pages 399-400).
You may be hard pressed to get relevant data, short of measuring the differences between two or more rifles of the same make & model, but of varying barrel length. Bullet weights and powder burn rates may also be significant factors in this determination, especially if reloads.
If you use good commercial or MILSPEC ammo, I don't believe you will find the differences between the 20" and 16" bbl significant enough to cause you worry about trajectory drop or kinetic energy retention within practical employment ranges.