[quote=Sage]
No no no no.
These guns don't work like earth guns either. Remember what you quoted from the OOC OP? How they use a fire core's energy to propel a bullet forward? How HARD could that be to make? I merely said that it wasn't shooting little bolts of fire, but rather, used the explosive energy of that fire to shoot the ballistic rounds. It's not that hard. Plasma rifles are the regular norm, and they are MUCH harder to make since they require core fusion to create plasma. On top of that, if you found a fire core, put it in a waterbottle, filled it up with bullets on top, and then activated bursts of the fire core's energy...Oh no! An assault rifle!
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The fire core propulsion only removes the one of simplest of the gun's moving parts and removes the explosiveness of bullets which are lying around. The hard part is getting the bullets in place. If you stick bullets in a tube and then have a controlled explosion in the bottom, you do not get an assault rifle. You instead get something which fires a handful of bullets all at once with no control. To deliver bullets in any weapon which takes more than one bullet per barrel needs some sort of feed, which generally requires well-crafted mechanisms. In manual guns, you still need a good mechanism to move the next bullet in place or it will get jammed. In semi-automatic and automatic weapons that mechanism also needs another mechanism to operate it (typically powered by the exhaust of the previous shot). These mechanisms are non-trivial. Their complexity could perhaps be comparable to a combustion engine, with all its rapidly moving parts. In no way beyond the technological capabilities of this world, but not readily available to just anyone, especially in a quarantined wasteland.

A weapon using a core, on the other hand, needs very minimal moving parts and cores can be found in just about everything, so many core weapons would be more readily available than ballistic firearms. Yes, fancy cores like plasma are not readily available, but lightning and fire and land are some of the most common cores around and could still make pretty decent weapons.

In short, it is quite easy to make a one-shot ballistic firearm, especially using cores to replace chemistry and ignition. But anything which fires more than one shot without reloading is much harder.
 
 
But these are gangs, not engineers. They wouldn't build their own guns. Like what Anriu said, they would be supplied their guns by gun dealers, which would have military connections. Unlike what Anriu said, after the apocalypse getting new guns would be all the more difficult because of the quarantine, martial law and destroyed economy. But this is a gang. They would have had stockpiles beforehand, then take weapons from other gangs. Same goes for the Golden Mercenaries, expect their pre-apocalypse connections were probably more legitimate than those of any gangs like the Blood Saints.