Individual barrels in a given caliber will vary significantly in bore diameter, length of throat, roughness, depth of grooves, and size of chamber. Sometimes this variance is so significant that a somewhat longer barrel will be slower than a shorter barrel with an identical load.
This difference, when commercial rifles are compared to the test barrels which are used to develop data by powder and bullet companies, is usually even more apparent. Test barrels are purposely made with minimum dimension chambers, minimal throats, and slick bores -- all of which is intended to keep their "maximum" loads under SAAMI standardized pressures when fired in barrels of unknown characteristics. And which usually achieve greater velocities than when fired in commercial barrels (of similar length).
When a given barrel starts out at, say, 26 inches, and is cut one inch and a time and chronographed after each cut, you can see the somewhat linear progression in velocities which is expected. But comparing a Winchester 22" .30-06 barrel to a Savage 24" .30-06 barrel will not necessarily tell you much about the effect of barrel length.
Another variable: Powder lots can and do vary, sometimes significantly (certain brands and types are particularly subject to lot-to-lot variations.) Also, the moisture content of the powder, which can actually vary from day to day, will have an effect (if only a small one) on the energy content of a give mass of powder, and therefore on pressure and velocity. "Duplicating" a load from a loading manual isn't as easy as it might sound; duplicating the performance of a load from a manual is even more elusive.
This difference, when commercial rifles are compared to the test barrels which are used to develop data by powder and bullet companies, is usually even more apparent. Test barrels are purposely made with minimum dimension chambers, minimal throats, and slick bores -- all of which is intended to keep their "maximum" loads under SAAMI standardized pressures when fired in barrels of unknown characteristics. And which usually achieve greater velocities than when fired in commercial barrels (of similar length).
When a given barrel starts out at, say, 26 inches, and is cut one inch and a time and chronographed after each cut, you can see the somewhat linear progression in velocities which is expected. But comparing a Winchester 22" .30-06 barrel to a Savage 24" .30-06 barrel will not necessarily tell you much about the effect of barrel length.
Another variable: Powder lots can and do vary, sometimes significantly (certain brands and types are particularly subject to lot-to-lot variations.) Also, the moisture content of the powder, which can actually vary from day to day, will have an effect (if only a small one) on the energy content of a give mass of powder, and therefore on pressure and velocity. "Duplicating" a load from a loading manual isn't as easy as it might sound; duplicating the performance of a load from a manual is even more elusive.