So, my question is: what common modern military equipment can survive
500+ years of continual use without becoming permanently unusable
[with repairs only at pre-industrial level]?
First point, you’re really making it hard for yourself by going for 500 years. For all but a handful of very durable materials, that is an extremely long time for anything to survive, and it doesn’t seem necessary for your plot. Not much actually useful stuff will survive that long. For example, as KerrAvon notes, if they are actually in “continual use” then after a lot less than 500 years, even bolt-action rifles will be completely shot-out. You can appreciably increase the list by reducing the time span. At 100 years, for example, there will be no people living who knew life before the collapse, but many carefully husbanded machines could still be operable. For example, there are plenty of examples of rifles a little over 100 years old that are still usable (if a long way from top condition), and even a few, rare examples of 100 year old motor vehicles that still work.
“Usable” is a little vague – I will assume you mean “usable for its original purpose or something close to it”, since otherwise, you can assume that all metals (and some other materials, like glass) will be reused and recycled indefinitely, just as has happened for most of the past 5,000 years.
“Pre-industrial” is also pretty vague. In terms of machine maintenance, there is a world of difference between a) a simple agrarian society where the artisans don’t know even the general principles behind engines; versus b) a developing nation where mechanics don’t have access to the best tools and parts but have a pretty good idea how to keep machinery running as long as possible with what they have. For example, in the real world there are developing nations that still have early Cold War armoured vehicles in their fleets. Most of the vehicles have been scrapped, and the remainder kept running by cannibalising the rest, but some 60 – 70 year old armoured vehicles are still in service. Immediately post-collapse, your society will be much more like b) than a). How it evolves over time is a difficult thing to predict; they may gradually recover tech, or gradually lose it. But by 500 years later, whatever direction the change goes, it is likely to be pretty extreme. Consider the Fall of Rome: in 500 years Europe went from a fairly advanced society, to the barbarism of the early Dark Ages, and then recovered to the High Middle Ages -- a very, very different society to Rome.
Anyway, I did a little brain storming, and here’s a few specific examples of things that might still be usable after 500 years of use:
- Body armour inserts: modern military body armour consists of a
mixture of high strength polymer cloth with critical points
reinforced with inserts usually made from super ceramics. The
polymers will not last anything like 500 years – apart from anything
else, they are slightly weakened every time they are exposed to
sunlight. However the ceramic plates are close to indestructible.
They can be cracked by intense impacts (such as bullet strikes), or
destroyed by extremely hot, sustained fires. Otherwise they will last
not 500 years but more like 50,000. How people actually use them when
the rest of the armour is gone, is up to your imagination.
- Identity discs (dog tags): deliberately designed to last an
extremely long time, and can probably survive for millennia. They
would not have the bearer’s name but that of a long-dead ancestor.
However to take an analogy from Dark Ages warriors after the Fall of
Rome, they might be seen as a badge of honourable descent from an
ancient hero. Of course one part of the set is supposed to be left in
the tomb of the hero, so the descendants would have only the graves
registry part. It might be an interesting episode to discover an
ancient tomb and find that your silvery tag of hardest mithril
matches that of the long dead hero!
- Stainless steel razor wire: Normal razor wire is either all
carbon steel, or carbon steel core with a stainless tape. Either of
these would have rusted to pieces in 500 years, unless installed
somewhere very arid. However there is also a grade rated for marine
exposure which is stainless throughout, and in any non-marine
environment it should be usable for centuries.
- Bayonets: utility knives will be worn out from re-sharpening,
but maybe not bayonets. For bayonets, continual use means daily wear,
not daily stabbings. In units where bayonets are not to be used as
utility knives, a bayonet only needs sharpening once every few years.
In that case it may last centuries, provided it is kept dry and
occasionally oiled. (However it isn't clear that a 21st century
bayonet would actually be more useful than a polearm purpose-built
from other recycled steel.)
- M1 steel helmets: I don’t know how long the new polymer based
ACH and LWH helmets will last, but they will most likely deteriorate
much like soft body armour. However the old school M1 is made from
high manganese Hadfield alloy. That is not quite as corrosion
resistant as stainless, but is much better than mild steel. With
modest care it could remain serviceable for centuries.
- (Parts of) Load bearing equipment: the modern polymer based
stuff again will not last. It is treated to resist the effects of
sunlight and could survive for a century or so if the stitching is
occasionally repaired, but 500 years seems dubious. Ironically, it is
once again the old-school stuff that will last better. It is made of
canvas and chemically dulled brass. The canvas is chemically treated
to make it very resistant to rotting, but it probably isn’t resistant
enough to last 500 years. However the brass fittings will last for
millennia, not centuries. If the fabric parts are replaced by leather
or new cloth whenever they wear out, you could end up with a hybrid
LBE designed to the new user’s needs, but incorporating old parts.
Still, the effort here may not be worthwhile.
- Optics: many optical devices are made of imperishable materials
and have no working parts as such. It is quite possible for these to
remain useful more or less indefinitely, provided they are kept
reverentially so they are not broken or scratched. This may include
field glasses (binoculars) and some types of rifle scopes. Of course,
once you let the lenses get scratched it is ruined.
- TNT: Most explosives are not very stable and can be stored at
most a few decades, usually less. High purity grades of TNT are very
much more stable, and it is not totally out of the question that
it could last 500 years if stored in a continuously cold place
such as a cave. (A bit of a guess, as no-one has ever tried that, but
there do already exist samples over 100 years old which seem to be
doing fine.) Detonators are a different question: they will not last
anything like that long, so new ones will need to be made, and that
White phosphorus:WP is a pure element and will last forever if protected from reacting with the external environment. There's the
problem: after decades, never mind hundreds of years, some of the
munitions holding the WP start to leak. Not a pretty look in the
middle of a magazine! However you could argue that in a professional
storage site, this should not set fire to the rest, and a reasonably
large fraction may still hold WP. Another issue is that most WP
munitions also have a small explosive "burster charge" which will
certainly have deteriorated, however not all munitions are stored
"fuzed" with this charge in them. Getting the WP out and using it is
another substantial challenge (quest?!) for Dark Ages warriors, or
perhaps apprentice mages in this case!
Tungsten penetrators: armour-piercing ammunition contains a super-hard core, or "penetrator", which is typically made from
tungsten heavy alloy or tungsten carbide. Your 26th century Dark Ages
warriors may not be able to launch these from a firearm, but they
could be re-purposed as the tips of crossbow bolts.
Radium dials: the tritium light sources used in some modern
military equipment has a half-life of 12 years, hence a practical
life of only 6 or 7 years between recharges. In contrast the (much
more dangerous) radium paint used on some World War 2 equipment has a
half-life of 1,600 years and will still be glowing brightly in your
Pan, set, messing and KFS: The mess pans are made of a
lightweight aluminium based alloy that is very resistant to
corrosion. They can be melted if allowed to boil dry, and will
eventually just wear out as the soft metal is scraped by harder
utensils. However I have a set that was made during the Vietnam War
and is still going strong. I would guess that with reasonable care, a
significant fraction of them would be usable for at least a few
centuries unless it was desired to recycle the metal for something
more valuable. As for the KFS (knife, fork, spoon set), they are a
tough grade of stainless steel and will last for millennia rather
than centuries. (In practice the biggest problem is losing part of
it, after which the rest doesn’t lock together properly and it
becomes easier to lose that also.)
Ammunition is not a problem, as they know how it make gunpowder and
shells, thanks to some diagrams they have
This is considerably more complicated than most folks seem to realise. Modern high performance rifle cartridges operate very close to the pressure limit of the steel, and are used to provide extreme accuracy (especially for snipers.) In other words, to make them simultaneously safe, high performance, accurate, and reliable is a high precision application for explosives: a careful balancing act between multiple processes. It isn’t a matter of following some internet recipe for nitrocellulose; powder making is way, way more complicated than that.
The process involves chemical solvents, precise quantities of various additives, and repeated chemical and physical testing. Quite minute amounts of batch-to-batch variation can result in greatly degraded accuracy, or even blowing up your gun. Slightly larger batch errors can produce very low performance (including low or inconsistent velocity at the same time as dangerously high pressures), misfires, hangfires, increased muzzle flash, and heavy gas system fouling that stops the weapon from cycling. It can even result in unstable powder that bursts into flame spontaneously and blows up your ammunition stores.
While it is plausible that your survivors could, with difficulty, make black powder for muzzle loaders, it is extremely unlikely that they could make smokeless powders for modern military rifles.