What is local to you for either service? Shipping may be expensive for either.
I like anno. Many interesting things can be done (splash, acid wash, ect...) I would PM 2slow4me (tonsixers) or bazookaboy. Both have great reputations and could answer specific questions.
I have seen some very nice powdercoat jobs. I would check out the KPCS thread
in the custom machinist thread. He does some translucient/sparkly finishes that look awesome.
I would ask about tolerences between the bolt and the body. Anno should not be a problem, but PC may be enough to mess up the gun.
good luck and keep us informed.