Shadowborn said he had done over 20 with his nerfed one using steel bolts, so that may be the case that all the + piercing damage bonuses are cancelling each other out. When EO had me testing Dribo's, on 200+ shots the damage was between 3 and 20 every time, which, unfortunately, didn't prove anything conclusively. Helaman said the +1 and +2 dwarven bolts stack with the un-nerfed machinists, so I'll just go that way next time.
As for rapid reload, Dribo can get 3 shots a round with her machinists, so it's working at least for the base 2 + haste. Not sure if it stacks with rapid shot until the patch is out, since I was told it doesn't right now but didn't want to blow the 2 feats to test it.