What is mostly exposed to salts generated by the primers is the chamber and bore, the breechface of the bolt, face of the piston and its surroundings under the handguard (where you see a sooty deposit) and the thread protector near the muzzle (I clean it and put some lithium heavy grease on the threads so this can just be wiped down with a wet cloth on the outside).
Trigger group isn't exposed. You can, however, flush it from time to time with some RemClean then RemOil the works.
I have made an adapter with a 303 British empty brass that I resized with a 7,62x39 die then drilled out to accept a soft copper tubing with an adapter which receives a standard funnel.
I take off the handguard, the piston and spring then hang the rifle over a sink, push the funnel adapter into the chamber and pour a quart of boiling water down the barrel.
I plug the muzzle with a small cork then pour another quart of boiling water so it shoots out of the gas port.
I wipe the remaining water off, flush the works with WD-40 to chase the moisture out then proceed to clean the chamber and barrel, swab the gas port and piston recess with conventional Ed's Red in normal fashion.
I clean the inside surface of the handguard with Ed's Red on a rag.
Piston and bolt get the boiling water treatment with a few drops of dishwashing detergent (Sunlight) followed with WD-40 then wiping and oiling.
I put some heavy lithium grease in the piston's groove before remounting.
That's about it.
PP.