Unrustle_Thine_Jimmies runs something similar. Without having tried it myself, I'd tend to agree with Hef: PDI 6.01mm barrel and the softest rubber you can find - either a Prometheus Purple or maybe look into one of those 40 deg rubbers they were selling on ehobbyasia.com and yeah, I'd probably go for R-Hop too, set up for .28g BB Bastards.
+ Wrap the forward part of the rubber in PTFE tape to get as tight a seal between the barrel, hop chamber, and rubber.
& Use a very good piston head O-Ring - Lees Precision Engineering are good (they have an eBay shop)
+ plenty of CT-2 Teflon grease on the O-ring and inside of cylinder (£4/50g search fleabay)
Adding weight to the gun also increases its inertia, which ought to improve accuracy by lessening involuntary movement while pulling the trigger...