(Sorry for the length, Barsoom is one of my interests.)
(Also, Don’t know if necromancy is frowned upon, looked at the guidelines and didn’t seem to have a cutoff date for posts, if there is I apologize and will keep my posts to recent threads.)
Um, Barsoomian Radium firearms aren’t like Civil War era firearms in any way, other than being projectile weapons. Not only does the Radium rifle out range them by hundreds of miles (200 effective, 300 maximum thanks to electric sights and finders), they throw more rounds down range than any man portable firearm Jasoom (Earth) has created that isn’t militarily compromised (Rifle based drum magazines) or purpose made (The various types of machine gun). Radium rifles autoload from a 200 round reservoir, cycling rounds fast enough that a single soldier can achieve suppression in ground combat, allowing their comrades to maneuver for the kill, and firing far enough that a serviceable anti-air envelope can be achieved by infantry, though not a comprehensive one thanks to the autoguns and cannon of Red Kingdom sky navies.
These advantages are helped by the type of bullet used in Barsoomian firearms, the Radium rifle fires a bullet made from a type of preternaturally (at least for us Jasoomians) strong Barsoomian glass. Inside the rounds hollow tip is contained a certain amount of crushed Radium. Depending on the size of the round , military regulations, or for those like the Panthans and nobles, personal preference, there can be more or less Radium, creating a bigger or smaller explosion. The rounds are manufactured, at least by the Red Martians, under artificial light as the lights used by the Reds are Radium and don’t cause the explosive reaction natural sunlight does. They are kept from exploding in the field by a sunlight blocking coating that, while perfectly durable for general field conditions, is stripped away by the extreme velocities (482803 m/s for long-arms, faster for the auto-guns and cannons of the airships of the Red Kingdoms’ fleets) that the bullets are traveling at and the heavy armor of enemy airships, allowing sunlight to hit the Radium. This can make the morning after night raids more harrowing than the attacks themselves.
Sadly, I haven’t got Phantoms of Mars yet, please know that I’m just trying to be helpful.
The harness holds a Barsoomians weapons. These are, for a Green Martian of the Thark (as an example):
1.) 1 Radium rifle (w/ ammunition)
2.) 1 Radium pistol (w/ ammunition)
3.) 1 long sword
4.) 1 short sword
5.) 1 hatchet
6.) 1 dagger
The harness also has bags for personal effects and equipment. This is explained in the equipment section of the JCOM rulebook (pg. 75 to be specific)
The cloak shades from the sun and protects from sand, it also keep a person warm during desert nights.
From the way JCOM’s rules seem to work, the listed equipment would be either flavor or a reason for the GM to call for a Momentum spend if the character didn’t have them when doing an action related to them. (Example, a character is making repairs on an airships engine but doesn’t have tools. The GM calls for a Momentum spend and the character happens to find some while rummaging around the engine room.)