Abstract We present deep Chandra, XMM–Newton, Giant Metrewave Radio Telescope, and Hα observations of the group–group merger NGC 6338. X-ray imaging and spectral mapping show that as well as trailing tails of cool, enriched gas, the two cool cores are embedded in an extensive region of shock-heated gas with temperatures rising to ∼5 keV. The velocity distribution of the member galaxies show that the merger is occurring primarily along the line of sight, and we estimate that the collision has produced shocks of Mach number $\mathcal {M}$ = 2.3 or greater, making this one of the most violent mergers yet observed between galaxy groups. Both cool cores host potential AGN cavities and Hα nebulae, indicating rapid radiative cooling. In the southern cool core around NGC 6338, we find that the X-ray filaments associated with the Hα nebula have low entropies (<10 keV cm2) and short cooling times (∼200–300 Myr). In the northern core, we identify an Hα cloud associated with a bar of dense, cool X-ray gas offset from the dominant galaxy. We find no evidence of current jet activity in either core. We estimate the total mass of the system and find that the product of this group–group merger will likely be a galaxy cluster.