Which... I'm on a phone and don't have sheets and all that handy, but I THINK you can actually make that work with twin Clan-Gauss. It would be a terrifying insta-bomb for any right torso crit, so it would be a BAD idea, but it's doable anyway. I think. Probably.
As if the 3L wasn't a bad idea ;D
Twin LGRs with LFE, no CASE. I love it anyway.
As noted earlier in this thread, the original Wizkids made stats for the Mangonel were apparently illegal, because you can't fit twin Clan Gauss to one torso without SFE (and SFE doesn't allow 5/8 mobility with that armament). Hence the approximate 3L, and the 3W "variant", which the TRO3085 describes as a practically different 'Mech rather than merely a variant.
Strictly speaking the later 4S variant more or less matches the Mangonel's intent, as HAGs are sometimes portrayed with multiple barrels. (Unfortunately for me, i don't really like HAGs, i'd rather just use missiles for most part.)