It really does not matter that much, these fairly loose smooth bores. You should not be getting dirt or crud much in your bore, maybe a little from the bb's in a dirty mag.
As long as the is no actual crud in there you are fine, On most airsoft rifles you have no good way to check anyway, with out disassembling your AEG. So pop a good blast of silicone in there from the feed end with the muzzle face down.
Then run some bb's through and you are good, just remember the bb's will go everywhere until things dry.
Hi performance sniper rifles are a little different, but in reality the same applies, if you mags are not full of dirt you should not have an issue.
Point in the post above about the hop is a good one, I literally clean my bore once a year.