All rifle barrels gain velocity per inch of added barrel length. If that were not true then you would not gain velocity as the barrel gets longer. 30 cal rifles and smaller gain more velocity per inch of barrel vs. big bore calibers, but they also gain velocity just not as much as quickly. For an example you take a 270 win in a 20" barrel and shoot a standard 3060 fps factory load it will only go about 2900 fps through a chronograph. Using a 24" barrel in the 270 win and same ammo it will go the 3060 fps because that was the length of most test barrels to generalize the 130 gr bullets that most run of the mill ammunition is loaded to. Using the same scenario and adding a 26" barrel to the mix the average velocity will be around 3140 fps using the same ammunition you started out with in all the other barrels. That's averaging 40fps per inch of barrel from 20" to 26". This I've tested myself through a chronograph using the same ammunition all in the same session with different barrel lengths. I did have a 28" barreled 270 win and that particular gun showed signs of pressure on the cases using some of the same loads that I had shot in my 26" barrel. I'm not sure just how long of a barrel would have to be to stop building pressure but I'm taking a safe bet you could go 36" before you created bullet drag in most modern fast big game calibers.

So the end result is you can build a lower pressure load and gain the same outcome of a hotter load by using a longer barrel plus it spreads the pressure out from the chamber throughout the barrel. Using a slower burning powder in this setup also creates more of a stable controlled explosion vs. a blast. Which in turn lowers the overall pressure of the load. When loaded for maximum loads in longer barreled rifles you do need to watch for signs of pressure.

All this is not needed to shoot long range or stabilize a bullet for great accuracy but at longer ranges added ft lbs for shooting game who wouldn't want to consider it. At 500 yards added fps is added ft lbs and less guess work on trajectory. It's all a + in my book. When I look at rifles I look at the ones with long barrels.