I agree about not liking the time limit of career, but the 1200 days is just what they score you on. During my one runthrough of career I didn't understand the Flashpoint system (this game really needed a manual), so I didn't visit half of the flashpoints until after day 1200, and finally tagged the last on them on day 1576. I play more for the roleplaying aspects, so it's important to me which factions I'm friendly or enemies with.
Thanks for the thoughtful comment.
One of the challenges with ammo weapons is having enough gun to justify carrying the ammo at all. A single SRM4 or, even worse, a 2, is pretty much dead weight; you want at least a 6, and ideally more, for your one ton of ammo. With LRMs you want at least ten tubes (and even then it's arguable that the tonnage is worth it, though if it's a 'mech's only source of stability damage it could very well be). With autocannon, a single AC/2 is a waste; you want at least a UAC/2 or two AC/2s. AC/5s and /10s are in an awkward spot, because one ton of ammo for a single AC/5 an awful lot, but for two AC/5s might not be enough (and two AC/5s is a pretty big tonnage investment all on its own). With /10s the problem is reversed: one ton of ammo for an AC/10 is a bit on the low side, but two is again an awful lot, and a more reasonable balance of, say, two guns to three tons of ammo, requires a tremendous amount of spare tonnage. AC/20s work very well with two tons per gun; ten AC/20 shots is going to cause problems for an opponent, especially if you're cautious about low-percentage shots.