When laser-etching channels through solid targets, the etch-rate is known to decrease with increasing depth, partly because of absorption at the sides of the channel. For ultrafast-laser pulses at repetition rates >100 MHz, we show that the etch-rate is also affected by optical properties of the beam: the channel acts as a waveguide, and so the pulses will decompose into dispersive normal modes. Additionally, plasma on the inner surface of the channel will cause scattering of the beam. These effects will cause a loss of spatial coherence in the pulse, which will affect the propagated intensity distribution and ultimately the etch-rate. We have characterized this effect for various foil thicknesses to determine the evolution of the beam while drilling through metal.