This has come up more than a few times on SD, and it’s obviously easier not to bother with all the hassle of cleaning. I'm driven by the fact that my rifles were so hard to find & get permission for that my plan is to keep each of them for 20 years or so. Others want to change their rifles more often, and there's always the option of passing them on after a bit of neglect to the next mug in the much wider right-hand market.
Every now and then when feeling idle I remember a very old article I found & photocopied entitled 'The Facts of Barrel Life (American Rifleman - Jan1978). Just dug it out again, and it says that any half-decent .308 barrel should keep its' accuracy for at least 4000 rounds, but very few do. The reason is that many shooters share the misconception that smokeless powder & non-corrosive primers make cleaning unnecessary.
It claims that rusting or corrosion from lack of cleaning is the most common cause of shortened barrel life. Damage from improper cleaning ranks next (thisisn’t defined, but cleaning without brushing is a common fault).
It then adds an extremely technical explanation over several pages of the process of corrosion within rifle barrels. This is disintegration of metal by electrochemical reaction or direct chemical attack.
The first is also called galvanic reaction, and occurs when different metals such as steel and copper with different electrical potentials are in close contact. The second occurs with the water vapour produced in the bore from combustion of a cartridge, and to some extent from atmospheric corrosion where oxygen, carbon dioxide, and water are always present.
To quote it directly - 'High power rifle barrels fired with gilding-metal jacketed bullets left uncleaned and un-oiled in an average urban atmosphere, are usually favourable to galvanic reaction.'
There are several paragraphs then, which I don't fully understand I admit, explaining how positively charged particles of barrel steel become ferrous ions, and negatively charged electrons react with the different metals of steel, copper, and zinc in the presence of water (electrolyte) to form hydroxyl ions. The ferrous & hydroxyl ions then combine to produce iron oxide(rust). Once rusting starts it's hard to spot (without a bore light & microscope I guess), and virtually impossible to stop or get rid of because it builds up as layers in the pits, crevices, and corners of rifling lands, under copper fouling, under dirt, and unburnt powder. It goes on to say that rust destroys accuracy. It raises pressures, and makes them more erratic. It deforms, and tears loose particles from the jacket rather than being pushed out or broken up from shot to shot.
It then says that the remedy to all this constant attack is simple: thorough cleaning following every shooting, and then a light protective oiling of the bore. Apparently jacket fouling is really difficult to remove completely with common gun cleaning solvents, but a thin wash does no real harm. The main thing is that any cleaning regime removes any loose powder fouling thus removing lodging places for moisture.
There's only one answer. The hard truth is that if you want to keep your shooting iron at it’s best for as long as possible then it's vital to clean it each & every time.