So far ... and as in SMG's and assault rifles.
Hardly as in chain guns or Mk.19's.
not true. you can legally own an m134 minigun in the US. as long as its pre-ban. and you probably need special connections to get the automatic weapons license. but private citizens do own them and they cost around $200,000.
I might go so far as to let them have improvised a sentry gun from a civilian-grade motion tracker and a semi-automatic weapon. But the official verdict has been that LV-426 is "a rock - no indigenous life", so the Company letting someone carry heavy military hardware, even obsolete, to the colony seems unlikely.
A planet not having indigenous life is still not going to stop gun nuts from bringing their guns. They will die before you separate them from their guns lol.
Again I dont think saying they have one aged sentry gun is an issue. If they had a stockpile of pulse rifles and grenade launchers, smartguns, and M3 body armor that would raise some red flags.