Wow, I struggled quite a bit to finde a build on the -5 which wasn't disappointing firepower-wise. Mounting dual Heavy Gauss was the obvious way to do, but my Annihilator 1X simply brings double the amount of ERMLs for additional punch while also having more armor. In addition to this it would mean ignoring the additional 2 ballistic hardpoints... Quad LBX with ECM was an option as well, but this wouldn't use the awesome Gauss quirks this variant has. Quad Light Gauss was funny for a moment, but still not what I was looking forward to be doing with an 100t mech. Finally an idea struck me. I already experimented with the Gauss + AC2 combo earlier during my MWO career for having exactly the same projectile speed. Putting two of each into the Fafnir finally made it 'klick' for me, as you can now deliver quite some suppression fire while also adding a nice 30 pinpoint strike while shaking your target's cockpit. Anecdotical evidence: