Tucker1965,
No, "the manufacturers" do not universally recommend break-in, and I know several manufacturers who suggest a break-in procedure only because so many of their customers demand one. These include both barrelmakers and custom riflemakers. And at least one barrel maker is happy when his customers do the one-shot-clean routine for dozens of rounds,, because it shortens barrel life by that many rounds, and they'll have to buy a new barrel sooner.
But here's why it doesn't really matter anyway, especially for hunting rifles:
1) The typical break-in method was developed by benchrest shooters who wanted their new barrels shooting as well as possible before the first match. But most hunters aren't getting their rifles ready for a match next weekend, and it won't make any significant difference in accuracy anyway.
2) Break-in supposedly works by smoothing the barrel, especially the reamer marks left in the throat. This is accomplished by shooting bullets through a super-clean bore, but the same thing happens if you just clean the bore down to bare steel between range sessions. Cleaning between every shot doesn't make break-in work any better, just faster--which is why benchrest shooters started doing it.
3) The bores of lapped barrels don't require break-in, just the throat.
4) If you really feel compelled to break-in a chamber throat quickly, it can accomplished more effectively and far quicker by firing 3 abrasive bullets, like those used in fire-lapping. Or you can wrap some fine steel wool around a bore-brush, then use a drill motor to spin the bore-brush in the chamber throat. More than one benchrest gunsmith does this.
5) If a factory barrel is very smooth (and many are these days) then the techniques outlined in #4 will break the throat in. If the bore isn't smooth, then you can do the one-shot-clean procedure for hundreds of rounds without making any significant difference--except perhaps burning out the throat. With rougher factory bores, it's far more effective to fire-lap or install Dyna-Tek Bore Coat--or both.
6) At least 90% of the guys I see "breaking in" their barrel at local ranges do it wrong. They fire a shot, then scrub the barrel with a bore-brush and a little liquid solvent, then push a patch or two through before firing another shot. This only gets some of the fouling out, but not all, and for break-in to really work, the throat area needs to be absolutely free of any fouling. Otherwise the bullets are just riding over the fouling, not bare steel.
But whatever makes you happy.
