Ed, YOU are the guy who has brought this idea home to so many people....... and it is a damned good idea. By default, if for no other reason, they are now, and henceforth shall be, "Ed's Famous O-Rings". BTW, I have been using this phrase now for a couple of years and no-one has called me on it, so it appears as if you are stuck with it! Thanks for the great idea; it may not have started off as your idea, but it seems to be yours now! If the idea startles you, just remember that Colt didn't invent the revolver, either!
John Sukey: Chohn, ze reason zat no-vun effer suzpects MOUSERS iss zat effeyvun KNOWS ze high qvality off Cherman Armaments, yes?.
By the time the Garand was built, tolerances and machine-tools had improved to the point of 100% total interchangeability among all of the 6.5 million M-1s built, regardless of year or factory. This was actually improved upon with the introduction of the M-14 rifle, which was the first industrial product in the world built on 100% CNC machine-tools. It was in the contract. Being that I live in a Free Country in which I am not allowed to shoot an American-made M-14 (nor the FAL which I spent years prepping for competition), I shoot a Communist Chinese copy thereof..... which has HAND-FINISHED parts. Go figure.
The American 1903 Springfield was touted loudly as the finest military rifle ever built in all the US-based gun magazines back when I was starting out. It is, quite obviously, based on the Mauser. I think the reasoning was that if the Springfield was the BEST, the Mauser had to be really GOOD, although generally it was mentioned that you really ought to check the headspace carefully, with the correct gauges. I think millions of sets of unnecessary gauges must have been sold from those articles.
Add to that that the actual gun manufacturers all were in the USA: Winchester, Colt, Remington, Marlin, Savage, Ithaca and so forth There WERE no ads from BSA or from Parker-Hale or Churchill of London because they could not get Dollars to pay for the ads, nor could they export Pounds Sterling at that time. So the American market, and manufacturers, were IT. Obvious point: flatter your local makers, most of whom had built equipment fr the US military in recent years.
But there also were big differences: US bolt rifles were OBSOLETE and so were being sold off. German bolt rifles were CAPTURED and Germany was only beginning to have the smallest pretence of an Army, much of which was being equipped with American-made (then Spanish, then finally German) autoloading (and/or automatic) rifles and so, again, German bolt rifles were obsolete.
In Britain it was different. Britain was worn-out and flat broke. If they got into another war, it would have to be with BOLT rifles. So they did the intelligent thing and made a bit of badly-needed foreign exchange by surplussing their OBSOLETE Bolt rifles, starting with the war-wearies which were not worth fixing up. Rifles for which they had active factories were FTRd and the last have only been released in the last 20 years. For several years, the only British military rifles available were in pretty rough condition. Once they had settled on the FAL (1957) they could surplus a few more Bolt rifles, these in better condition.
So for several years you had Lee-Enfield rifles on the market, sold dirt cheap, which were not in the finest of nick and were ancient to boot. Add to this the SAAMI specs (which do NOT agree with Board of Ordnance specs) and you have rifles which have not seen the inside of a repair depot since Lawrence of Arabia lost them, firing badly out-of-spec ammunition, separating casings merrily and blasting their too-small-diameter bullets madly downrange in an effort to actually hit something. Obviously, there were problems, but nobody knew how to cure them because the "experts" were designing the ammunition so, obviously, the problem was that crappy British $12 rifle. "Hmm.... looks like a HEADSPACE problem; better gauge it." But in these cases, many times it WAS a headspace problem..... but it was generated by OUT OF SPEC ammunition, not by the Rifles themselves, which would have shot a lot better if the ammo had been up to spec.
The REALLY GOOD part of it all is that people like you and I managed to pick up some pretty rare rifles for junk price, as witness my $12 Navy 1907 Mark I*** SMLE. You probably have a treasure or two of your own with a similar absurd price-tag.
One thing I do remember rather vividly is a magazine article done by Col. Charles Askins, comparing the main rifles of the two World Wars. In conclusion, he stated, "We Americans build a target rifle, the Germans build a sporting rifle and the British just make something to kill people with." Being a penurious Canadian newbie to the hobby, I took rather a perverse pride in this evaluation which later had sound reasons to back it up. I do know that my first 98 Mauser cost 3 times what I paid for a full-wood SMLE..... and I still, 50 years later, can't afford a 1903 Springfield!
Hope this helps.