Another issue that hasn't been mentioned here, is the tolerances which are held in manufacturing the barrel. What may appear to be wear, may actually be tolerances in boring the barrel, cutting rifling, threading cutting, and other variances in manufacturing.

It's been mentioned also in other fora, that forcing cones aren't always milled square to the cylinder face. There's also the factor about the amount of off-center entry to the forcing cone caused by a cylinder, case, and bullet which are not in perfect alignment.

While heat-generating pressure surely creates friction, and there is a decided lack of lubrication in the barrel, it would take some very detailed and extensive testing to determine what the real cause of barrel wear is.