I feel that standard modern day firearms will not be very popular in the GFL era.  My reasoning is that you would need something that will work in various atmospheres.  For a modern firearm to function you must have oxygen for the propellant to ignite and burn and in many planets atmo's the oxygen content may not be high enough to function.  I feel that compressed nitrogen/oxygen weapons would be an alternative.  These "needle" guns would fire a projectile like a depleted uranium spike or smaller caliber needle.  The range would be minimal but for a sidearm that is ok.  The mass behind the projectile would penetrate many types of armor and it would pack a punch like Mike Tyson.  Don't forget we are talking about the richest sentients in the galaxy so I don't think money would be an issue, maybe energy weapons would be feasable for them.  Traditional firearms would probably still be the norm for lower class sentients as they would not have the money to acquire high tech gear such as compressed air weapons or energy weapons.  Just my thoughts.

 It's all fun and games until the nukes start falling and the zombies start calling...

I would like an official ruling on the tech, but this is a great place to start. Thanks a lot!

First, you have some super-cool points.

Second, remember that the oxygen levels are consistent enough across the worlds of the five GFL species that those species can operate on each of those five planets without life-support equipment. I am ignorant of physics in the most part, but that would seem to address your firearm concern at least on Human, Ki and Sklorno worlds, as well as in all Quyth domes and most Quyth planets. 

Third, there is very strict weapons control in Quyth domes. The more expensive/advanced a weapon is to manufacture, odds are the easier it is to track and possibly detect. Depleted uranium, for example, still emits radiation, just at a lower level. Weapons detectors would likely pick these up, providing a suspect and therefore a trail to trace back to the weapons smugglers. A simple revolver with metals that do not emit radiation (or emit lower radiation) on the other hand, wouldn't be as detectable. 

Fourth, a gun is a gun is a gun. While more advanced firearms are surely coming our way, I don't see a far-future world where we get rid of something that does a great job at blowing holes in bodies.

Thoughts? 

Basically, part of one of my characters is that he's borderline obsessed with Ancient-Earth Old West, so he has old-modeled revolvers which, I assume, are unusual for the times and much louder and more obnoxious to the people around him when he decides it's time to do some shooting. In essence, my question is: Would having an old-timey type revolver be so out-of-place that it(and the sounds it makes) are nearly unrecognizable unless you had studied ancient Earth? I mean, it would still be recognizable as a gun, but might be strange enough that what type of fun it might be would be difficulkt to discern. And also if they would even work, I was thinking if it wouldn't work it wouldn't be too difficult to modify it to stay true to the loudness and all the other fun stuff about old revolvers, but still work on the planets/in the domes he would be operating in. 

You nailed most of what I wanted to know, thanks a lot!

The GFL rules regulate the operational environment of the stadiums, but beyond that, isn't it kinda up for grabs? For instance, the Whitokians, Dolphins and Phibbies all operate underwater, and surely have weapons that work in that environment. On the Quyth worlds, usually the bubble that surrounds the stadium supports all the lifeforms, but beyond the bubble is irradiated wasteland unsuitable for humans (at least on a few of the Quyth planets).

I kinda got the impression that the humans were the weak link in the chain, and that the Quyth, Ki, Sklorno and Creterakians all could adapt better to hostile environs that would be fatal for humans. But I guess you're saying that the general biosphere supports all the lifeforms? 

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Gutter Sistren whipping boy, innoventor of words, Life Coach to the Damned.

Gunpowder based firearms require no atmospheric oxygen to function.  The gunpowder contains it's own oxidizer and will function perfectly even in a vacuum.  As long as the atmosphere in question doesn't compromise the metal of the weapon or the integrity of the cartridges or gunpowder, it will fire just fine.
Ve veri veniversum vivus vici (idea’s cannot be killed)

Stadiums operate within specified parameters. However, those ranges were set because they encapsulated existing conditions on the main football-playing worlds. A human can live on Sklorno, Ki, and some Quyth worlds. Alien planets may be stinky, but they are largely survivable if you don't mind vomiting a bunch. 

I did some further research and I was incorrect in saying that a modern firearm needs atmospheric O2 to function.  I still think that weapons will progress away from "gunpowder" but who knows.  We have become so good at creating weapons to punch holes in things that it's hard to imagine where it will go to next. 

It's all fun and games until the nukes start falling and the zombies start calling...

First:  Thank you.

Second:  I was incorrect in saying a modern "gunpowder" weapon won't funtion without O2.  The O2 is already present in the cartridge and therefore would work just fine. 

Third:  Many modern pistols are synthetic frames with minimal metal and can be smuggled through metal detectors, not easy but it can be done.  I have handled ceramic knives and I can tell you they are far sharper and more dangerous than their steel brothers.  Undetectable unless you are patted down or x-rayed and even then depending on how it's concealed it could be missed. 

Fourth: Well said, I have been told that many old Marines were still qualifying on the old Springfield M1903 even though it had been dropped from service for the M1 Garand and eventually the M-16.  The oldies are still goodies.

It's all fun and games until the nukes start falling and the zombies start calling...

I was incorrect about gunpowder and needing external O2 but I think the rest was still valid.  I can't wait to hear your storyline!

It's all fun and games until the nukes start falling and the zombies start calling...