At one time Winchester had Model 97 shotguns that had been used for in-house testing of production shotshells. Some of these had been used for years, firing over a million rounds while remaining reliable. During World War II the Army used some sort of wing shooting to train aerial gunnery. I recall that the better quality American doubles were used initially, also Winchester Model 12s, Remington Model 11 semiautomatics. Probably many other production repeaters were also used. Double guns broke down early in the gunnery training. Repeaters pretty much lasted forever, firing hundreds of thousands of rounds

For generations Americans preferred Marlin and Winchester lever action rifles, buying millions upon millions for sporting use, a much smaller number used by law enforcement. I realize that firing stress on shotguns is significantly less than rifles using smokeless powder center fire lever action ammunition. Since lever actions of the black powder era had older quality control and metallurgy, let's not include them in my query.

And finally to the query: did either in-house testing or user shooting demonstrate anything even close to this sort of reliable durability?

My father was a Bombardier with the Army Air Corps on B-24 heavy bombers in WWII, with 30 missions over France and NAZI Germany.
Upon completion of his last mission. he was re-assigned to USAAF bases in New Mexico, utilizing the Winchester M-97 and M-12 for aerial gunnery training
for air crews. It helped a great deal.

I have a few 1886 nickel steel rifles in 33 WCF made up until the 1930s. They still are reliable and function well. Well enough to have been used by hunters like Ben Lilly for decades, and are still being used. These are 100 yr-old rifles, still functioning.

Strong enough for Winchester to continue the design
in their Model 71 rifle.

I have a 123 year old Winchester Model 1894 .30 W.C.F. lever action repeating rifle.

One look at it and another look down the bore and you can tell it’s been used often and hard.

It still works like a charm and is plenty accurate enough for hunting purposes.

I know it’s a sample of one, but as you say, there are millions more of them still in use out there all over the world.

John Browning was truly a genius.