Boyle's Law assumes the gas is in a state of equilibrium. The correct equation is PVT, where T is the absolute temperature (Kelvin). It also assumes ideal gas behavior, which requires no interaction between gases in a mixture. Ignition in a firearm meets none of these criteria.

The critical factor for reloading is peak pressure, which is dependent on the rate of conflagration. That rate is strongly dependent on pressure, temperature, and characteristics of the powder, including composition, shape and surface area (which changes in time). A flake powder is likely to burn faster than extruded powder, and extruded powders often have perforations to increase the initial rate of burning, and maintaining that rate since the surface area decreases as the rate of burning increases.

It is fairly straight forward to express this process with differential equations, but those equations can't be solved - there is no explicit solution. If you wonder why scientists can't predict the weather, it is precisely for the same reason. The equations can't be solved without assumptions and gross simplification. Astronomers can't predict the next asteroid to destroy life on earth (or even streak across the sky) because there is no solution to 3 body gravitation. (How, then, is Global Warming "settled science"?)

That said, the best way to predict the pressure is to measure it, many times under controlled conditions, to produce an estimate with an estimated margin of error, then load on the safe side of that estimate.