Years ago I went thru a dozen rifles a month.....not all mine...many was bought to my range as needing work....many needed bedding and others just sighting in.

I tried a lot of "break in" things and in the end just couldn't conclusively say anything about it. This much I can say.....those that went thru the break in practice actually spent time at the range and most new rifles are never exposed to any range.....a few shots off the truck fender and off to the woods hunting.

Those that actually spent time at the range developing loads, working up different loads, getting the bedding right, glass if needed were far better off than the others. There was absolutely no comparison. However any advantage was never conclusively attributed to the break in efforts.

Of the guns I personally own about a third of them actually saw a concerted break in attempt. None of them are measurably better than the others.