I think this is sensible. We are in an "after the apocalyps" scenario here. I highly doubt a lot of weapons factories survived the nuclear holocaust and of those few that _did_ survive, I am sure none was engaged in developing new types of guns. Therefor, sticking with "real life" guns seems to be sensible.
Of course, for the fancy, high end weapons (they WILL be in, now won´t they?) like Meson Canons and Ion Beamer, names have to be made up (I realy would like for them to have decent names, not just Meson Canon but, I don´t know, Rheinmetall MAC-2000 Meson Assault Cannon or something like that)
Actually the Ranger Centre itself was producing light weapons etc (pistols). And the Base Cochise was producing whole robotic soldiers etc, so weapon production wouldn't be that difficult to explain away.
Making effective weapons is actually not that complex if you have the tools. Look at WW2, weapon production in places like Australia was important, because our usual suppliers were obviously dedicated elsewhere. A good example is the "Owen" sub machine gun.