As usual useful TVTropes:
Comes in at least six flavors:
Break Out the Museum Piece: It doesn't matter that there hasn't been any fighting for a long time. Their weapons are safely stored away and always ready if needed.
OK, the real question is how to implement it. The situation so far:
- tech level is similar to contemporary (except maybe slightly better semi-autonomous vehicles)
- because of savage cost cutting... uhm... doctrine there is a tendency to give up tanks and jet aircraft (wheeled artillery, wheeled IFV or turboprop drones are fine; nevertheless there should be anti-tank and anti-aircraft weapons)
- the procurement policy is terribly un-American ;) it does not have to be top tech, but has to be cheap to be bought in high quantity
- its a post-post-apocaliptic setting, where some survivors managed to teleport to different planets. There is no contact with other human groups, and technically speaking its only indirectly inferred that other groups survived.
- the expected role of the army is to be able to deter, beat or at least inflict unacceptable loses to any high tech Cortes wannabe or any low tech hordes that hypothetically may one day open some dimensional gate (As budget committee explained: it even does not have to win, it would be enough if cost of conquering is clearly higher than loot)
- because of low population the technological progress is slow
- the main defence idea is the follow - low spending on military in each year, but concentrating money on long term investment (like stashed weapons or fortifications), so after ~50 years having actually quite impressive defence capabilities
- the ability to be stored with no or little maintenance is a key design requirement from the start and some trade offs would be accepted to achieve that
This NOT a question "which way to store gun?". The question is only concerning main and most blatant design features that under such requirements would have to be done differently (like for example, something in line: how to address issue that modern bullet proof vest with 5 year shelf life or what to do about rubber parts in any vehicle)
CLARIFICATIONS [after reading comments]:
I don't assume literally being able to store weapons for infinity, "should survive a few centuries but requires new easily replaceable battery every ten years" is close enough for all practical purposes
storage environment - as good as realistically possible, so presumably dry, cold, stable temperature vaults. If specific storage problem can be solved by ex. keeping in oxygen free conditions or -20 C degrees that would also be achievable.
it does not matter how aggressive local fauna is, but one way or another, at least 90% of armament just after production would end up being stored waiting for some potential Ragnarok.
the question covers which most significant things would have to be redesigned in order to allow for such storage. (For example, if I understand correctly hand held guns should be more or less fine, as I've read that Americans used some WW2 ammo during Desert Storm. If that's correct - rifles are not problematic in overall picture)
For story purposes it matters how their armament would be different at first day of a conflict, because they had to sacrifice some weapon capabilities for purpose of good storage. For example: guns were be OK, but their bullet proof vest were much too bulky to be of much use.