In our world, when the world breaks down because of an influx of mythological creatures(Krakens, giants, etc...) sea travel becomes nigh but impossible because of people dying on their ships and serving as food and the ships sunk to seabeds. Power plants and grids get destroyed beyond easy repair. A total blackout.
So no new oil will be shipped for burning, nor will new coal come from other sources and long distance travel generally comes dangerous because of creatures roaming the countryside. Most ore mines have long since closed or are heavily dependent on a functioning power network for air circulation and water pumps, so they are out of the running soon too.
In a scenario where all public power means have broken down, no more gas is being pumped around, waterworks has broken down and people are being plucked off by an influx of monsters.
How long can the military keep using modern firearms, assuming it has to be used sparingly but daily to ward off monsters, protect convoys and remaining fuel supplies and power generators.
I assume bullets, gunpowder manufacture, replacement parts production, maintenance oil production and cleaning materials production basically stop as soon as the power and the employees stop showing up to deal with the power/water/food/monster crisis. Not to mention the question where to get the raw materials for gun powder, replacement part manufacture in the quantities needed.
The threat of monsters came fast and quickly overwhelmed the entire world collapsing everything in a blackout. All new attempts to restore any sort of power grid will be attacked soon, so only local heavily defended power generation is possible, but even that draws regular attacks on those generators. Day by day local infratructure like powerlines and windmills and solar farms get attacked by creatures rendering them inoperable and scrap.
I assume there are some doomsday scenarios lying around, and that the military can probably crank out a manufacturing division. But how long could the military keep on functioning with modern firearms, and what would a possible next step fallback be for the military? Would they resort to bows or are there other "backups" when supplies run out?