The average velocity loss when the SAME barrel is shortened is around 25 fps per inch. This has been demonstrated by many experiments over the years--including miine.
But different barrels often (perhaps even usually) have slightly different chamber dimensions, bore/groove dimensions, throat lengths and rifling angles, etc. etc. As a result, some longer barrels won't achieve much--or any--"extra" velocity with the same loads.
Another point: As with "improved" versions of various standard rounds, many handloaders get more velocity out of longer barrels because they work up loads by looking at traditional "pressure signs," such as primer appearance, bolt lift, ejector-hole marks on the case head, etc. These can be caused by other things, but also often don't show up until pressures are around 70,000 or even 75.000 PSI--far higher than any SAAMI pressures.
This means that most of the extra velocity is often not due to the extra case capacity of an improved round, or 2-4" of barrel length, or (especially) a tiny bit of extra "volume" is some brass. Instead it's due to pushing brass to its limits--which isn't very safe.
