The increasing average power of ultrashort-pulse lasers requires efficient use of the available pulse energy, and MHz burst operation offers one route by distributing this energy among temporally separated sub-pulses. However, systematic understanding of how ablation efficiency evolves with the number of pulses per burst, Np, during percussion drilling of thin metal foils remains limited. Therefore, we investigate how the maximum depth-based ablation efficiency, ηmax, evolves with Np up to 40 during MHz burst-mode percussion drilling of 50-µm-thick metal foils using 300-fs, 1030-nm laser pulses. Measurements for stainless steel (SS) and Ti are compared with those for Al and Cu, with thermal diffusivities approximately one order of magnitude higher. We separate ηmax into two contributions: the optimum sub-pulse fluence, Fbopt, and the average ablation depth per sub-pulse, Δdb. A simple theoretical model describes how residual heat accumulation affects Fbopt, while the logarithmic ablation-depth law is extended phenomenologically to describe deviations from additive depth scaling. For SS and Ti, Fbopt decreases with Np, in agreement with the baseline predicted by the model, while moderately super-additive depth scaling produces a strong initial increase in ηmax; at higher Np, ηmax reaches more than 470% above the single-pulse value. Al and Cu deviate from this baseline through an initial increase in Fbopt and pronounced odd-even modulation of Δdb and ηmax. These departures are consistent with additional plume-mediated attenuation and re-deposition reported in the literature. At larger Np, the decrease in Δdb and increasingly melt-dominated morphology coincide with diminishing further gains in ηmax, consistent with melt redistribution and plume-mediated attenuation.
