Here is a quick and dirty rebuild of your mech that keeps the spirit of what you were trying to do, but improves the application -
smurfy link
As others already mentioned, ERPPCs are a no-go on a build like that. Way too hot, and the Atlas is a dismal sniper. Besides, you are using the LBX20 set-up, investing over 22 tons +ammo into it, you want to get closer to bring that firepower to bear and justify the tonnage. Switch to LLAS. Way less hot, lighter, and work closer to the range of your main weapon system.
With the tonnage saved there, drop 1 ton of ammo for both the LBX and Streaks to fit in a 325STD engine and a few extra heatsinks. You're still slow, but now you have a chance at re-positioning and keeping up with mechs so you can use those LBXs. You also have the sinks to fire multiple times now, bonus.
You still have 30 trigger pulls for the combined LBX20, that is plenty. 100 streaks will go a long way when fired 6 at a time. That's 16 and a half trigger pulls. Considering you are only going to want to use streaks against lights to shoo them away (using them against big targets is not an effective use of heat in a brawl, better off using the cannons and lasers), that is way more than enough at 1 ton, your original 2 tons was complete overkill.
I think that is a complete improvement over your current set-up without fundamentally changing what you wanted to do. If you are not married to the LBX20 package, I might recommend the AC20. I think a pinpoint 20 damage strike is better than the crit-seeking spread out 20 damage of the LBX.
You also can fit a 350 engine and even MORE heatsinks in with that build That said, the dual shotty build is fun.