It's not as simple as just muzzle pressure. Noise measurements are higher in the direction the barrel is pointing than from behind. Partly this is due to the angle the bullet's supersonic shock wave propagates. A German paper I found a while back points to pre-war modeling which assumes sound begins radiating once the front of a gas sphere formed by the muzzle exhaust slows to sub sonic. Said gas ball would be affected by charge weight, powder energy density and burning speed, expansion ratio, etc.
If your company or college has an ISO subscription, ISO CEN 17201-1 provides a way to estimate peak sound. I don't have access (too cheap to pay for it), but I'd be happy to read a copy you send me.
