Pressure does NOT equal velocity. Some powders produce higher pressure to achieve the same velocity as others.
There are a lot of things that can affect pressure.
Temperature, altitude, chambers, bores, and i'm sure there are others also.
Example of temp: Friend of mine had a near book max load he developed for a rifle in early spring, no pressure signs. In July he went out with same load to do some penetration testing, primers blew out and it was all he could do to open the bolt.
And yes there are slow and fast barrels.
2 of the Reminton 700 30-06's in my house shoot the same load of IMR 4350 in R-P cases, with the same primer, with a 165 gr Hornady. One of them gets 2720 fps the other right at 2900 fps, and they both have 22" barrels. The load shoots accurately out of both rifles.
