Sage is correct, Even if at the start of the RP society had little to no guns other than the standard flint lock pistols. Core weaponry was most likely in testing phases.

Now we skip ahead a year, the world has literally gone to shit and the demand for advanced weaponry has risen substantially. IE: The military mass produced core weaponry that had only been in testing phases a year earlier, and now uses it as standard issue.

Gangs got a hold of some of it through a black market of some kind most likely, and the GM's probably have contacts within the military itself, or have some way of producing this equipment on sight. Either way the appearance of 'guns' is not really an issue.

The only real problem I have at the moment I have already stated prior to this post :).

I hope this helps resolve some issues :).