Yeah if you clean out your weapon, maintain it by doing things like oiling it up it will last. The Mosin Nagant as I've seen can take one hell of a beating and still preform perfectly fine, won't jam on you if I recall correctly too, super reliable.
I can personally attest that this is not correct. When I was in Iraq, we were shooting .50BMG that was made in the 1940's ('43 or '45, IIRC). Well-made cartridges will last a long time as long as they are kept from corroding so much that the casing falls apart. 200 years is still probably pushing it, however.
Wasn't it implied that the Commonwealth's progression was unintentionally stunted by the hands of the Institute? I mean, their super mutants wrecking every caravan and town, and then their synths wiping out settlements like University Point for a single component (University Point was arguably one of the most potentially advanced settlements from the looks of it). Blacksmiths might've popped up in some of the better settlements, but then got taken out by one of the Institute's creation. So they destroy settlements left and right looking for small components, pre-war parts, records, and escaped synths, plus they accidentally let loose another strain of FEV. All in all, there probably wasn't much chance for anyone to advance far enough to attempt smithing. Or at least someone did, but then them and everything around them got burnt down because the Institute needed another sensor module. Whoops. Still, it's only an assumption. But considering how many places get wiped out by synths, yet the Institute remaining only a rumour, it just seems obvious that they just destroy places completely, whether they intend to or not. Isn't ammunition easier to produce than firearms themselves? There are plenty of repurpose-able factories and industrial buildings in the Commonwealth. Sure, none of them was actually being used (gotta have more feral ghoul dungeons ), but when it comes down to it, if say the Gunners needed ammunition they could just invade one and start repurposing it. I'm not actually sure what precise equipment you would need for ammunition production though, but even then, it's the United States and finding the remains of a gun factory shouldn't be too hard. I'm very confident that the Gunners, who use guns and turrets like they're going out of style, can't just pull military-level amounts of ammunition out from their asses. Bethesda tried to imply there were more complex works behind everything, but didn't actually want to show it, so as to save the time and effort of actually doing the "show not tell" games are supposed to do. Functional ammo manufacturing and weapon production probably exists, conveniently "located" outside the game map where we can't access it.
The weapon actually reminds me of a M1917 machine gun with the iron sights of an anti aircraft gun. I wouldn't ever use that POS. I would rather battle rifle using full caliber ammunition like a 30.06 HCAR before I use that bulky monstrosity.