The answer is a resounding "sort of, but not really." Let me explain in my usual roundabout way.
This setting's one-sentence elevator pitch is "Imagine the movie The Road Warrior, with flintlocks and horses instead of shotguns and V-8 Interceptors." If I get to expand to two sentences, I add on, "The Lord Humungus is eight feet tall, purple, and smells like brimstone," but really the first sentence is the relevant one here.
|still dressed the same, though.|
Think back to the last time you watched The Road Warrior. How many people do you remember getting shot with guns in it?
Not many. In fact, among the rampaging neo-barbarians of the Lord Humungus' army, he's the only one with a gun at all, and he's only got five bullets to his name.
How's that work?
In the universe of The Road Warrior, World War III occurred and was fought as a purely conventional war, without recourse to nuclear weapons -- and it was an extremely drawn-out, protracted conflict that exhausted the resources of everyone involved and lead to the slow, creeping decline of civilization we see in The Road Warrior's predecessor, Mad Max.
Without an overwhelming catastrophe like a nuclear exchange, the products of civilization continue to exist even though civilization itself has fallen apart, and are there to be scavenged and repurposed. That's why everyone in The Road Warrior is wearing piecemeal armor made out of leather jackets, football pads, scrap metal, i.e., whatever they could scrounge up and put together. In fact, watching the film, apart from the centrality of vehicular combat, most of the fight scenes have a decidedly medieval vibe to them, as people fight with crossbows and melee weapons. Guns are now luxury weapons, highly-sought-after for their lethality but no longer really in production; after all, any post-apocalyptic average joe can hammer nails into a club, but producing gunpowder requires specialized knowledge, tools and materials.
|this asshole notwithstanding.|
So while in the Once-Green and Pleasant Land, there are still all sorts of muskets and pistols floating around from before the Cataclysm, and people who either still remember how to make them or can figure out how to reverse engineer a flintlock mechanism, the number of people producing gunpowder has become very small, and is probably shrinking every year. A sealed cask of black powder, then, would be a treasure to fight and die over, as valuable, if not more so, then a +1 sword.