Fafnir
The FNR-5 Fafnir is an Inner Sphere Assault BattleMech known for essentially being two fuckoff huge guns on legs. Woe betide any smaller BattleMech in the Battletech world that gets into range of this thing.
Overview
The flagship BattleMech of Defiance Industries after their self-named Defiance project got associated with Operation EXCALIBUR, aka Brandal Gareth's failed Free Star Republic coup, the Fafnir was designed to have the biggest guns around without going into artillery territory. To this end it was built mainly to support two "Thor's Hammers"/"Hammerfists" Heavy Gauss Cannons - weapons that were a recent innovation of the Lyran Commonwealth at its time of creation in 3063. Just to haul and power these things requires a special Foundation Type 220 Endo Steel chassis capable of supporting both the guns and the mech's 19 tons of Durallex Special Heavy V2 armor while still being lightweight, as well as a Vlar 300 fusion engine that also lets it reach a top speed of 54 kilometers an hour.
A Mechwarrior that pilots one of these things can expect to commit ungodly amounts of rape at range, since those Hammerfists are the largest non-artillery guns period and thus can do an XBAWKSHUEG amount of damage in a short timeframe. This makes the 'Mech more suited for long-range engagements. It also really benefits from being near a supply line, since without one to keep its 4 tons of Thor's Hammers ammo stocked it'll only have two ER Medium Lasers and a Medium Pulse laser once the Gausses are dry.