We evaluated the problem of modeling the decay of the primary pulse amplitudes of air-gun clusters caused by the traditional assumption of sphericality. This was done by generalizing the Rayleigh equation to work with arbitrary bubble shapes, while retaining the assumption of incompressibility. To approximate the coalescence of the bubbles, we let the shapes be isosurfaces of the velocity potential. With this method, it is possible to model the firing of clustered air guns at any separation distance, including small distances that would cause two spherical bubbles to overlap. In this way, we obtained results matching the relative decay shown to be present for air-gun clusters. In addition, this method also allowed a way of calibrating the model such that effects created by the presence of the gun, compared to just a single spherical air bubble, may be estimated and included.