No, not OOC for Xenos to leave Liberty space. The RP on the bases indicates that they can jump into neighboring systems as attack strikes may call for it. (ie., Xeno based in Kepler could jump up to Shikoku to attack Samura Industries)
But, these attacks are fairly uncommon due to logistics of long range support. Fighters should stay close to friendly bases, and the Xenos aren't exactly welcomed out there.
As for Nomad guns, I use them on Apocolypse. John, the pilot, spent nearly a year traversing Sirius space with Meiun, a Zoner, and collected some rather unique items during that time abroad.