My take is that the majority of muzzle corrosion is due to simple humidity. We stand our rifles up on their buttends, muzzle upwards. Moisture will enter the bore, and collect on the inside. Rarely will it descend much further. So it sits there month after month, eating the metal little by little. Keeping our bores oiled, or even greased for longterm storage, will prevent this.
Those poor rods get the blame for a lot of damage.....
I do clean from the chamber end if possible, but many designs don't allow this. I have seen some Ruger 10-22 owners actually drill an access hole in the receiver rear to allow a rod through. My old BSA International has this feature, and I suspect the Ruger guys got the idea from BSA. Just drop the trigger pack and the bolt, and clean her good!
I only use brushes for the first cleaning of a new/old rifle, when I get it. After that, a simple swipe with oiled patches keeps her clean.