I am in the process of building an AR15 in .204 for myself. Just finished machining my lower. I was going to just do the typical Flat black, but then looked over at all my custom acidwash/splashed paintball guns I have, I decided I wanted my AR to to have an acidwash/splash/sponge anno. I asked on one of the gun forums i'm on, and they were less than helpfull.
Honestly, I would go with type III dust black, but you should do what you want if you went to all the trouble of machining and building your own.
.204 Ruger huh?! Should be a fun (and fast) plinker.
Old school splashed would be my second choice.
Exercise your constitutional rights, but keep your laws of of mine.