The decrease in chamber pressure as the gas expands down the barrel of a firearm is not a simple linear relationship. It depends on various factors such as the design of the firearm, type of ammunition, barrel length, and other variables.
In a typical firearm, the chamber pressure initially spikes when the primer ignites the propellant, and then it rapidly rises as the burning propellant generates gases. As the bullet travels down the barrel, the pressure gradually decreases as the expanding gases do work on the bullet, accelerating it forward.
The exact percentage-wise rate of pressure decrease along the barrel is difficult to determine without specific information about the firearm and ammunition being used. It can vary depending on factors such as the powder burn rate, bullet weight, bullet design, and barrel characteristics.
At the muzzle, the pressure is still significant, but it is generally lower than the peak pressure in the chamber. The pressure at the muzzle depends on factors such as barrel length, bullet weight, and bullet design. The pressure at the muzzle is typically measured in units such as pounds per square inch (psi) or pascals (Pa).
It's important to note that the specific pressure values and their distribution along the barrel are typically not disclosed by firearm manufacturers and are proprietary information. The behavior of chamber pressure and gas expansion in firearms is a complex topic that is extensively studied by ballisticians and engineers.