I am wondering what determines a 'Rare Loot" it seems to me the Gravis is not rare at all, it is actually rare to sell one, being lvl 100 maxed weapons, and those who are lvl 100 probably wouldn't use a Gravis. Here are some stats, for Gravis Drops 336 GLR's 333 GLP's 389 GBR's 248 GBP's I've looted 20 and sold a couple in the early days, and really, 1306 Gravis to date is not rare eh! Hopefully at some stage they can be made SIB, by upgrading them or something similar.
Well some Calypso counterparts are rare Like the glr33 , which Essentially is a justifier mk5. Or the glp36, which has the m2930 me as a counterpart. Or the counterpart of the as 147 me. Good weapons and the Calypso versions don't come cheap.