Re: 5.45x39 AR-15 parts?
The only ready to go GP upper I know in this caliber is the Adams Arms. A friend bought one and I've shot it quite a bit, it just runs and runs without needing to be dripping wet. He went with it because pretty much all he runs is corrosive and he has no problems although he's rather "enthusiastic" when cleaning. Disassembles everything and throws it in the kitchen sink, then pours a tea kettle through the chamber out the muzzle onto everything else in the sink.
I will say this, there's no sign of corrosion. He just CLPs it when everything is dry. The only downside is that it's not as accurate as I've "heard" some are. He's been running a scope I had out on loan so it may have bit the dust along the way. To come up with one with a fluted barrel, I think you're going to have to go to somebody like Scott Milk on this site.