We must live with the fact that different barrels give different pressure and velocity responses to the "same" loads.

Even industrial test barrels made to the same spec will give different responses. That's why the industry uses the elaborate "Proof Ammo" procedure. The assessed pressure and velocity of a given lot of proof ammo reflects the "average" response obtained when fired in different test barrels made to the same specification.

Your mileage may differ.

As it was explained to me many years ago, "I feel sorry for those who believe that ballistics is an exact science. They just don't understand the problems."