Hang back and snipe with the GR at the beginning of a match. Jumpsniping is difficult, but can be done. Ammo is plenty, so don't hesitate spamming Gauss slugs around. 2x MLas and 2x SRM4 with a lot of ammo are very potent secondary weapons, especially when used on targets hit by your GR previously. As a small bonus the XL275 coming with the mech can be used here. You could also go with the XL280 which wheighs the same if you've got a spare one. I found this to be a very potent loadout, which isn't exclusive on the 5M, but can be used on the 2D2 as well. Variant - SSRMs instead of SRMs This is a very strong variant to the basic loadout. BAP not only helps while sniping with the GR, but also supplements the SSRMs. I reduced the engine size because speed isn't that important with the proposed playstyle.
Fiddled around with all kind of builds, but ended up with an almost similar one at the end. This thing is a monster, easily capable of dealing 600+ dmg a drop.
The thing I like the most about the Gauss is how easy it is to keep a low profile with it. No cockpit shake. no muzzle fire, no smoke. If you sit in the back, the projectile speed will let you land your shots with relative ease, without them even noticing you most of the time, being busy blinded by the fireworks the rest of your team is putting out with all the colorful lasers and banging ACs. If they ever manage to close up on you, you can also put up quite a brawl with the SRMs, lasers, and even the Gauss, since it has no dead range. I've tried most of the builds possible, spread out over the 3 chassis, but this one is by far the easiest to consistently deal significant dmg for me.
It is probably just me, but I can never get Gauss+XL engines to work. When they DO notice me and I get missiles on me or someone sees the gauss protuding from my side, I go down extremely quickly. I guess it's great for the people who are less conspicuous than me in a match, I'll stick to a UAC5+2LL and some srms for my loadout. also very good at pinging from range but much less volatile.
Can drop .5t armor for a CASE no prob, if you find yourself killed by gauss explosion too often. Happened to me once ever so far. P.S.: Nvm, CASE wont help
This is a very potent variant to the basic loadout. BAP not only helps while sniping with the GR, but also supplements the SSRMs. I reduced the engine size because speed isn't that important with the playstyle proposed in the OP.
Using 50 volleys of SSRM is not gonna happen if you're staying a bit on the background as you would want to with an exploding gun next to your xl. the extra speed DOES come in handy as you can disengage much harder than the XL255 variant can, and even chase lights that run from your streaks to some extent.
Every time I see these without a PPC in the arm, I go for them, as an easy kill. 20 damage every gauss explosion makes you an easy kill.
I was going to post this build but it's already here. Solid build for PUG matches where you can't really trust your mates to keep the enemy at range, the weapon combination lets you fight at all ranges. Prefer to use the SRM4s though, more punch to fight the heavier mechs.
