...can't help but wonder what a civilian colony would be doing with sentry guns in the first place ... especially bearing in mind that they obviously, according to the movie, don't have access to military hardware, instead needing to improvise and employ seismic survey charges as weapons...
In my game, that sentry gun simply isn't there.
It said it was deployed by the colonial militia. the colonial militia could conceivably have a sentry gun.
If there is a militia at the colony, it seems to be equipped with civilian hardware only (which would make sense, actually - what would such a militia expect to deal with that a shotgun or a pistol can't handle?).
Militias by definition consist of civilians. Even when they're an officially recognised force and not simply irregulars, they don't get the really advanced or powerful military stuff, or equipment that require specialised training. And one would think that if they'd got sentry guns, they would certainly
have got grenades, and mines ... and from what the movie tells us, this does not seem to have been the case.
For all we know, it was the militia that used the survey charges as their heavy ordnance.
Just "getting" a sentry gun seems to me akin to me just "getting" a Vulcan CIWS. Not likely, I'd say...