To be honest, the inner barrel that comes with them is pretty good. If its accuracy that you feel is lacking, the biggest improvement i made with mine was putting the Tridos CNC hop arm in it - it made a dramatic improvement.
I presume that you have discounted fluctuating gas pressures as being the cause of inconsistency? Until i got an airtac shotgun shell adapter mag + the tridos arm for mine, it was barely useable. Now, with those changes, it is utterly fantastic.