Robot: TL 8 Sentry Gun

I would think that an Aliens-style sentry gun would more properly be TL 10.
The Alien franchise does not use TL so it is hard to say what equivalent it would be in Traveller. The Alien sentry gun was used more as an example. But feel free to create this sentry gun with TL10.
Their low berths are better than ours.

Suspect that the Colonial Marine combat equipment is more designed for ruggedness and reliability, than cutting edge.

Starfaring performance comparatively is rather fuzzy, between Alien(s) and Traveller.