Bleach is great. A solution of around a 1 1/2 cups per gallon of water is strong enough kill many of the really harmfull pathogens. You can fill a spray bottle with it and spray it leave it for 5 minutes and rinse or you can fill the barrell up with water add at a lower amount and let it run off.
ME I love the stuff and add a whole lot of it but its doubtfull you'll need that much if the barrell was food grade.