Naah, I was using one of the lighter assaults. Forgot which one though. (The heavies can mount some of those fast PPCs, but I don't like the heat bar appearing red in Mech Lab.)
The two Railgun-mounting giants were awesome, but whenever I went in 1v1 duel with my friend, he'd either bring in a Cougar just to spite me or one of his many Daishi tanks; whereas I could out-snipe him with a good mouse, he could rain me from far off, behind a cliff with reasonable accuracy.
LBX10 Madcat... sounds familiar (Never played online, think I toyed with a variant like that once but I ended up on the receiving end of an Atlas' "****-you" attack on that round), but did you have space for any other weapons...?