79: I'm sure you know all this,but barrels vary all over the map.
Some are tight, some are loose,some are rough and some are smooth. They have different numbers of lands,different throat lengths,different materials. Even how the lands are configured can affect how the bullet engages the lands and affect velocity.
All these variables can conspire together to increase or decrease resistance that the bullet encounters in traveling down the bore. Mix in different powder lots, bearing surface of different bullets,materials used,etc and it's no small wonder we see differences in the velocity from the same cartridge and bullet, and same load,from one rifle to another.And why we can even run into the pressure walls without getting the velocity we expect of a cartridge.
If we hand load enough, we have all bumped into this stuff at one time or another.
The one variable that seems to overcome all the others IMHO is case capacity,and even then we get into issues of pressures that we mostly all just take educated guesses at reading. But the point is that even though we don't alway get everything we want from a cartridge, we will generally get more than with any case that's smaller and holds less powder.
One chap (buddy) I know who had a Mashburn said that his 168 gr Barnes did not get the 3200 fps others seemed to be getting with 160 gr bullets.Velocities were barely over 3100 fps. I speculated that was because it only weighed 7 gr less than a 175 gr Partition,and maybe had even more bearing surface.
I never worked with the Barnes in the Mashburn but for yuks sometime, measure a 160 Partition.....it ain't .284.(or at least it was not the last time I measured one)...Hint.
