I definitely would take the COR gun over the RNG crossbow, as RNG already has several powerful non-relic options available, while COR is basically Martial, Coffinmaker, or bust.
Semi-related: I was going to do RNG after PLD, the obvious route would be to go for Marksmanship since I will have 8/8 merits from COR, but I just inherited an EBow for my 37 RNG.
Check that, it appears I just inherited a 75 RNG as well.
I mean cool, but it would be nice if all my old-school friends didn't quit for WoW and such.
