The question of what is a reasonable lifetime is a very important issue. The buyer wants an MTBF of 1,000,000 years with an infinite lifetime, for $20.00. Thirty bucks if the gun is shiny. The manufacturer has very different expectations. Now any darn fool can build a twenty or thirty pound single shot firearm that never breaks, the real art is making one as light as possible.
From what I have read, the early M1911's had to pass a 6,000 round endurance test. To pass a 6,000 round endurance test, obviously the pistol has to be built so all pass this test, which means the pistol will function longer on the average. These numbers probably came from the experience of the military and estimates of a reasonable number of rounds per year. It could have come from the Arsenals and Shooting Teams, where round counts, pistols rebuilt, etc, were considered. I believe a case can be made that the firearm should last as long as the barrel, as barrels typically wear out first. So with the soft steels of the day, after 5000 or 6000 rounds the barrels would be shot out, the pistol send back to Depot for overhaul. Once at Depot any part could be replaced if need be.
Something to note, the early M1911's were made of plain carbon steels and were not even heat treated. Gunsmiths state that the parts are dead soft. By the time WW2 rolls around, from what I have heard, slides and frames are still plain carbon steels but case hardened for wear. They will last a bit longer.
I got to shoot with the All Guard, they are using WW2 era frames, maybe other parts, in their Bullseye Pistols. I talked with the Armorer and forgot all the details other than these slides and frames have to be refitted in time spans equal to years. He had wear criteria and I forgot what it was, I think it was frame rail thickness. Frame rails are peened, then the slide and frame are assembled with a hammer and grinding compound in the slide rails. The slide is beat back and forth till experience says, the fit is just right. Not too tight and not loose at all.
I also talked to the AMU shooters. I asked about pistol durability, etc. Seems their frames and slides were made by Caspian Arms. No one on the firing line knew how old they were other than these things had been around before anyone joined the team. Some guys thought they might have been made in the 1970s or 80's. I talked to the gunsmith David Sams about these pistols, he claims he helped set them up in the middle 1990's.
I asked the AMU shooters how many rounds they shoot through the things, well the AMU shooter said about 5,000 to 7,000 rounds. I asked, "per year"?, no, "per month!"
BONG!
AMU shooters wore out triggers, sears, in time periods close to a year, if my recollection was right. Barrels took several years. Springs were replaced frequently. No one had ever seen a frame or slide crack or break, and none had every required refitting, within the experience (around six years max) of the shooters on the line. That is just exceptional.
There are a number of differences between vintage M1911's and these modern era pistols. The first is alloy steels. Alloy steels are so superior to the plain carbon steels that were in exclusive use up through WW2 in firearms. When arms manufacturer's used nickel steel, such as Winchester, they bragged about it. After WW2, even though that was vacuum tube era, everyone knew and understood that the material properties of alloys were so superior that only legacy designs and lazy firearms manufacturers used plain carbon steels in structural elements. Something else changed in firearms manufacturing in the 1990's, and that was the semi conductor revolution. CNC machining has produced the tightest factory pistols that have ever been made. Pick up a Kimber, a Range Master, or a Les Baer. They don't rattle. The old Colt series 70's and 80's rattled when new. The combination of better steels, cleaner steels, and better machining have extended the lifetime of firearms, given the same loads.
Barrels are still going to be a limiting element, they do wear internally. I have no idea of the fatigue life of a barrel, they are a pressure vessel, I expect at some point they will rupture. A low pressure cartridge such as a 45 ACP I expect the barrel will never fatigue rupture before the rifling is worn smooth. However, I am aware of a shooter who had a vintage 30-06 barrel rebored to 35 Whelen. The rifle barrel had gone through its normal service life, was old steel, and yet the barrel was enlarged internally, making it weaker, and installed on the action. The shooter fired a couple of factory 35 Whelen rounds, the barrel burst, and the owner is mad at the barrel maker. The barrel maker blaims the owner and ammunition, the owner blaims the barrel maker.
The owner should have had enough sense to have a new barrel installed, but the barrel maker is the one who is assumed to know better. He has the product liability. Manufacturer's are assumed to be expert in these things. Anyone reboring old barrels is taking a risk. You don't know the quality of the old steel, you don't know what loads were fired down the things. The owner might have been an Ackleyite, one of those worship able types that believe that firearms don't have structural limits because P.O. Ackley said so. Ackley said no one knew what loads firearms were designed to, therefore it was fine and dandy to fire his non pressure tested loads, which actually are around 80,000, to 90,000 psia, down firearms that were designed for 50,000 psia.