The Japanese Samurai were enthusiastic proponents of firearms. One of the most dramatic scenes in a Japanese movie is the ending of Kagemusha, which recreates the Battle of Nagashino, where the Takeda clan was essentially destroyed as a power by the effects of volley fire by Oda Nobunaga's troops.

Volley fire isn't fun if you are running into it

So if firearms were so effective, why did they essentially disappear from the Japanese arsenal during the Tokugawa shogunate? The reason is cultural.

Samurai, like European knights or Ottoman Janissaries were highly skilled warriors who started their military training in boyhood and were not required (indeed forbidden) to engage in trade or crafts. With a lifetime of training and the ability to practice their arts on a daily basis, they were fearsome warriors who could defeat any opponents who were not skilled or equipped the same way they were. Peasant armies were little more than rabbles or (at best) spear carriers who could form a line to slow down opponents as you got organized or regrouped.

Firearms were the key to the "Infantry Revolution" which started in the mid to late 1400's, as weapons and tactics were developed which let large numbers of unskilled men take to the field and effectively fight against Samurai, Knights, Janissaries or others who's prowess depended on long training and practice. Firearms swept away Knights and Janissaries in Europe and the Middle East, but Japan, being much more isolated and insular, was able to block the importation of firearms and forbid the local manufacture, allowing the Samurai class to retain its power and not fear being overthrown by a peasant revolt.

So in your setting, there must be some cultural reason to fear or forbid the manufacture or development of firearms. If the apocalyptic event took place and required large numbers of people to shelter in makeshift structures for protection from the environment, then there might be a well founded cultural fear of firearms, since using firearms in a flimsy shelter could cause a breach and allow deadly radiation or whatever other environmental effects caused the apocalypse to enter. After several generations, any pre existing firearms would probably have deteriorated to the point they don't work anyway, ammunition wold have decayed and no one in their right mind would even consider recreating them.

So the point isn't to create some sort of mind bending alteration of the laws of physics (preventing the chemical reactions that cause explosions probably means invalidating the chemical reactions needed for life to exist), but to explain why people don't make firearms anymore.

The Japanese Samurai were enthusiastic proponents of firearms. One of the most dramatic scenes in a Japanese movie is the ending of Kagemusha, which recreates the Battle of Nagashino, where the Takeda clan was essentially destroyed as a power by the effects of volley fire by Oda Nobunaga's troops.

Volley fire isn't fun if you are running into it

So if firearms were so effective, why did they essentially disappear from the Japanese arsenal during the Tokugawa shogunate? The reason is cultural.

Samurai, like European knights or Ottoman Janissaries were highly skilled warriors who started their military training in boyhood and were not required (indeed forbidden) to engage in trade or crafts. With a lifetime of training and the ability to practice their arts on a daily basis, they were fearsome warriors who could defeat any opponents who were not skilled or equipped the same way they were. Peasant armies were little more than rabbles or (at best) spear carriers who could form a line to slow down opponents as you got organized or regrouped.

Firearms were the key to the "Infantry Revolution" which started in the mid to late 1400's, as weapons and tactics were developed which let large numbers of unskilled men take to the field and effectively fight against Samurai, Knights, Janissaries or others who's prowess depended on long training and practice. Firearms swept away Knights and Janissaries in Europe and the Middle East, but Japan, being much more isolated and insular, was able to block the importation of firearms and forbid the local manufacture, allowing the Samurai class to retain its power and not fear being overthrown by a peasant revolt.

So in your setting, there must be some cultural reason to fear or forbid the manufacture or development of firearms. If the apocalyptic event took place and required large numbers of people to shelter in makeshift structures for protection from the environment, then there might be a well founded cultural fear of firearms, since using firearms in a flimsy shelter could cause a breach and allow deadly radiation or whatever other environmental effects caused the apocalypse to enter. After several generations, any pre existing firearms would probably have deteriorated to the point they don't work anyway, ammunition would have decayed and no one in their right mind would even consider recreating them.

So the point isn't to create some sort of mind bending alteration of the laws of physics (preventing the chemical reactions that cause explosions probably means invalidating the chemical reactions needed for life to exist), but to explain why people don't make firearms anymore.