You can replace the BAP with a case for the Gausee Rifle and 1 ton of ammo of your choice. Another option is to mix/replace LLas and ERLLas. If you don't use CC then replace that for an extra slot, you can add 1 ton of ammo and pump up the engine rating.
@enileph, here's an old blast from the past. I really think this is one of the silliest builds for an Atlas I've ever seen. Not enough LRMs, and too many ERLL.
Damn that is a bad build. That LRM5... *cringe* No case for Gauss... CC, ammo placement, slow 300 engine... That and that is pre-ghostheat, with the 4x LL. Just remember, those were the days where noisy cricket (4x PPC cicada) works and I was runing things with Xl Stalker with 6x PPC/ERLL. A more modern version, with SRM instead and downgraded to ML.