Velocity losses vary by cartridge case volume and bore diameter. A small case with a large bore ie 35 Rem will not get perfomance improvements with additional barrel length beyond 20 Inches. The powder is going to burn quickley and the large bore will displace a lot of volume by the time bullet reaches the end of the barrel. That means the pressure levels in the barrel will be very low and the bullet will no longer be accelerating.

A 7MM ultra mag will have lots of powder to burn,and the bullet travelling down the barrel will displace less volume in an equal length barrel relative to the 35 cal barrel. This means very high barrel pressures at 20 inches and an accerating bullet.

The best quantification of this I have seen is a test using a 7mm-08 with a 24 inch barrel. The gun was fired repeatedly and an average velocity was measured. The barrel was cut to 20" and retested. The velocity only decreased by 60 FPS total or 15 FPS per Inch. I feel this is the best way to test because it eliminates variables between barrels which often timeshas more to do with the velocity improvements than the lenght. Many of the velocity increases from the high end custom barrels is due to reduced friction in the finely finished barrel not the additional 2 inches of barrel length. Most buyers will mistakenly attribute all of the velocity increase to the length instead of the finish.