In my setting I need to have a dungeon where people cannot use guns or other gunpowder weapons.
The reason can be anything: it's impossible to bring guns in, guns don't work inside the dungeon, using a gun is unsafe for the shooter, anything.
I originally thought about a flammable gas being released in the dungeon. But I'm not sure if gunshots can ignite gasses in the air, and if such gasses exist that would be breathable (I want no other side effects).
Is there any way a modern civilization could build such a dungeon?
**Edit- ** more details, as requested
The setting The setting is an extreme reality show. Contestants are put on an island and they try to get points that are hidden in dungeons scattered around. There are also native people on the island which can supply them with guns, which is why I don't want guns to be used inside the dungeons- the show runners want gory face-to-face combat in case two teams arrive at the same time.
I don't want to get into the nature of the natives in this post, so for the sake of the question assume they have modern level technology but are easily manipulated and don't exactly have free will. I'm still not sure about the number of natives there will be, but probably the population of a small city.
The dungeon The dungeon is built by an incredibly rich entertainment company for the reality show. They can be assumed to have almost unlimited resources. The technology level is like I said, near future. The company however cannot have employees physically on the island while the show is running, so the "no guns" rule should be enforced physically and not by some guard. I thought about having the natives guard the dungeons, but native guards can be bribed or killed. The dungeon should be able to last about 2 months. However maintenance visits are possible.
The purpose of the dungeons are to create tense conflict between contestants if they arrive at the same time (which is likely, because teaming with others is encouraged by the rules but so are betrayals). The system should also not allow older kind of guns. Weapons inside should mostly be things like spears, knives, bows, etc'. If the system can be cheated by a smart contestant that's fine, but it shouldn't be easy.
The people entering the dungeon The people who are supposed to enter the dungeon are the reality show contestants. There are 100 of them, and because of the natives, they are capable of manufacturing modern-day items.
They would have time to prepare but not as much as they want, because they will try to get to the dungeon first before other contestant teams. The natives will tell them what will happen if they use guns in the dungeon, but not why.