Quick and effective method for me is to take the stocks off and, using an aerosol synthetic oil and a spray extension, spray it up inside the mechanism. Almost certainly, the oil will get into every crack and crevice protecting the parts and lubricating as well. Leave it set overnight and the vehicle part of the spray will evaporate, leaving only the lightest protective coating inside.
Of course I am almost exclusively buying 1900 to pre-WWII guns so will always strip out all parts, clean and inspect them before reassembly. It takes a long time for the interior to get dirty and synthetic oil will not cause varnish build-up over the years, unlike some petroleum based oils & greases.