people seem to be using the AC20 way too much on atlases. here's something to counter that trend. http://mwo.smurfy-net.de/mechlab#i=18&l=e0339a9477911fcad709bad9961924aa7db3fd0d the main weapons are the large lasers and the LBX10, which can be happily spammed for a long time due to having 4 tons of ammo. the ERPPCs also provide a decent sniping capability.
here's an alternate AC10 version, removing a ton of ammo to make it fit: http://mwo.smurfy-net.de/mechlab#i=18&l=5e5f6a970f88809ab351ff1d38a5b1363ad681db