In cosmological first-order phase transitions (PT) with relativistic bubble walls, high-energy shells of particles generically form on the inner and outer sides of the walls. Shells from different bubbles can then collide with energies much larger than the PT or inflation scales, and with sizeable rates, realising a `bubbletron'. As an application, we calculate the maximal dark matter mass $M_{DM}$ that can be produced from shell collisions in a U(1) gauge PT, for scales of the PT $v_\varphi$ from MeV to $10^{16}$ GeV. We find for example $M_{DM} \sim 10^6/10^{11}/10^{15}$ GeV for $v_\varphi \sim 10^{-2}/10^3/10^8$ GeV. The gravity wave signal sourced at the PT then links Pulsar Timing Arrays with the PeV scale, LISA with the ZeV one, and the Einstein Telescope with grand unification.