How do you fit the biggest gun in the smallest possible mech? By getting a big richard. The smallest possible package that can mount a long tom, with extensive research and design to allow such a thing to even happen!
6mm Scale Battlemech compatible with the most popular mech based tabletop wargames.
Models available cheaper, and earlier, on my patreon at https://www.patreon.com/LelanianIndustrialComplex or my tribes here on MMF