I am also a mechanical engineer with a little metallurgy experience.

A 'clean' rifle barrel is kind of an oxymoron. Once bullets are fired then erosion begins, especially in the throat. Added to that are the tooling marks left by normal production processes. So, there is rarely a clean, smooth surface for the bullet to travel through (hand lapped custom barrels are better in this regard). If you really want to see how bad it is get a good borescope and look into a bore. Also, let's consider that cleaning with many solvents just removes powder fouling and leaves copper in the bore. To remove the copper you need to use a product that will do that.

So, what we look for is consistency and that may be different if you are a hunter or paper shooter. As a hunter I used to sight in barrels by cleaning well between each shot, presenting a bore that will be the same when I take that first shot. Yes, follow up shots are done as well, just in case, and any POI change was noted between clean and fouled barrels. For paper punching I shoot a couple of shots before going for a bullseye to make sure that the record shots are on the same kind of conditions.

Also consider what happens in the bore. The first round will encounter only whatever lube you left in the barrel. Following will have burned powder and copper residue to deal with. That residue will partly be expelled by the next bullet and partly 'ironed' into the pits in the bore. Some will claim that enough of this makes the bore 'smooth' by 'filling' the pits and is good for the barrel. If you look at some of the data taken by precision shooters they have plotted group sizes based on how many rounds they have fired since a cleaning with copper removal. There is a definite change with the number of rounds fired between cleanings with copper removal.

So, all that is why some people clean a lot and some don't clean very much. One other factor is that very frequent cleaning may cause more barrel wear than shooting due to the cleaning rod rubbing on the bore. Hunting rifles that are fired only a few times between cleaning are probably worn more by the rod than the bullet. Some precision shooters will clean often and just replace the barrel more often. Some of those may replace a barrel every year or even sooner depending on the cartridge they are shooting.