At the end of the season, for an extended interval after firing or if in rainy weather.
I've done this with the barrels in the stock or remove., with or without scopes.
From the breach plug end... Remove and clean thoroughly the breach plug. When the breach plug is dry, I oil the entire plug and palce in a small plastic bag. I don't lleave the breach plug installed for extended storage. Personal preferance. I also leave the actions "open" on all my weapons as well for air circulation.
Clean with a patch and bore solvent a couple of times such as "Butch's Bore Shine" to remove any plastic, copper or lead and fouling.
Then a dry patch.
Then I flush with hot water unitl the entire barrel is hot to the touch and then wire brush several strokes and then again with the hot water until the entire barrel is very hot to the touch.
Then, 2-3 dry patches.
Then, a couple of alcohol patches.
Followed with another dry patch.
Wait about 10 minutes or so.
Then, oil patchwith your favorite lube. I use "Otis U85, but I also use "Break Free".
Then after the barrel has completely cooled in an hour or so, I run anther oiled patch through to catch any residual moisture and apply a final coat of protectant.
Good until next year or the next time to be used.
Then before shooting, 2-3 alcohol patches followed with a couple of dry patches to get the bore really clean and dry.
Load and go shooting.
IMHO...
Best,