The ammo might seem excessive, and it probably is, but this is designed to pop off LRM after LRM without afterthought. The MPLs are for close-in-support, and the TAG and BAP are there for obvious reasons. 13T of ammo for 2xALRM15 is 78 shots. I estimate that's around 4 minutes of sustained fire, once you factor in quirks, modules, and mech skills.