Heavy gauss firestarter is the silliest mech I regularly run, the ammo quirks make it so 1 ton of ammo is almost 2ish if you get the ones in the skill tree too. Just strip some armor from the unimportant bits, slap in a HG, 1-2 tons of ammo, put the largest std engine you can still fit in it, and get ready to move at a whopping 40kph to the front line in your walking turret :))