Keep in mind that WD-40 is not actually a lubricant it is a solvent and should be treated as such. If you want to clean your inner barrel with WD-40 your welcome too, but keep it away from your hopup. Run a cleaning pad with a light spray of WD-40 on it through, then run clean dry pads through until they come out clean.
Doing so will keep it clean but if you over do it could result in the hopup failing eventually.
Also keep in mind that a barrel has no moving parts it does not require lubrication, just proper cleaning and maintenance to ensure that any residue as well as dirt and debris are cleaned out.