So what exactly is so special about the damage caused by the holorifle? Does it just do bonus damage against ghost men or is there something else? Anyone try it out against non-ghost men targets yet?

it seems to be a variation of the tesla cannon causes hard direct dge and aoe/on going damage

The Holorifle is advanced tech according to Elijah. Seems likely to me that it's a prototype weapon of some sort, so who knows what it is?