In Burroughs' Barsoom novels, he justifies all the swordfights (when his heroes have access to some rather brutal firearms) by having all Martians abide by a code of honor that would make it unthinkable to anyone but the lowest rogue to shoot someone who has a sword drawn. The problem with doing this in a role-playing game, of course, is that most players are rat bastards who would have their character shoot a guy in that situation without blinking an eye.
On some level, that's just common sense. Maybe that's why firearms are either not part of the typical landscape of other planetary romances (like in Carter's Green Star and Callisto novels) or of reduced effectiveness (Howard's Almuric). I have tended Athanor toward the latter approach, because I like the swashbuckling idea of having some pistols around, especially if they are essentially black powder arms.
But Athanor also is home to many pieces of lost technology, including a variety of firearms. But in this case, we're talking rayguns, Buck Rogers/Flash Gordon style ray guns. Call them lasers or death rays, I don't really care. But ray guns will be rare but dangerous treasure that heroes can get ahold of, and will make guns much more likely to be used. And the PCs are going to use them with brutal efficiency.
So will I enforce some sort of Burroughsian "code of chivalry?" Of course not. There are better ways to control this through more subtle Referee manipulation:
1. Control the Ammunition
How much can players recharge their weapon? If the answer is not much or not at all, every shot from the precious 4d6 damage laser pistol is going to be tracked much more carefully.
2. No Mercy Cuts Both Ways
An arms race of brutal behavior will eventually either land the PCs in trouble or kill them. Combined withe some in- or out-of character discussion of this escalation may help curb some of it.
3. Actions Have Consequences
Ruthless cowards will not make as many friends as brave warriors. Though the players could always live by the motto "oderint dum metuant" I suppose.
However, I think that this stuff isn't necessary-- at least not in a heavy-handed way. My experience is that powerful items so long as they don't dominate all aspects of play and plot can work well in the game if they ar treated realistically. Others will change their expectations (for good or ill) of the guy who can fell a Tyrannosaur with a couple of shots. Having such an item may make you the target of the attentions of Bad People who want to take it from you, from Good People who want your help, and from Scared People who find your power intimidating. Include that kind of stuff in game, and things get interesting.