But that’s precisely my point: according to the game and the adventure, the colonists did have access to such hardware, with sentry guns, pulse rifles, and whatnot.
The information we get from the actual movie does not support such an assumption, as per the previous discussion.
I, along with some others, and our games, stick with the movie. Others are free to make other choices. And as you illustrate, one can find arguments to justify either position.
To be picky here... the movie doesn't state either way.
Apone says "Sir, you copying this? Looks like hits from small arms fire... uuuh, with some explosives damage. It's probably seismic survey charges."
That's ALL the movie tells us. Sgt. Apone's hasty on the spot analysis of the damages right inside the entrance.
That is the only mention of the colonist's defensive capabilities we hear of.
Now, the marines entered through the north lock, and proceeded southward (on both first and second floor).
Then Gorman, Burke, Ripley and Bishop entered through the south lock.
Nobody went to the sublevels.
So... what do
we know from the film?
1: There was small arms fire. That includes assault rifles, so the M41 is certainly possible.
2: There were explosions. Apone didn't specifically identify them (which would be hard to do without thorough analysis) so just guessed at seismic charges.
3: Nobody goes down to the sublevels during the movie. So there's no evidence to existence or non-existence of the sentry gun.
Keeping in mind that the "securing" of the building seemed rushed and sloppy (Gorman sends his troops through one part of the entire complex and decides it's safe) and the fact that they're not really spending all that long in there before they find the colonists PDT's, it's very possible that there's a Sentry Gun and an M41 puls rifle lying in the sublevels.
Furthermore, the Sentry Gun would most likely have been destroyed by the Aliens before the Marines arrive, since it's been several days and the Aliens have been back and forth several times to seemingly remove all the bodies from the area. They would have discovered the Sentry gun before that and since it wasn't in a sealed area, the Aliens could very well have figured out how to bypass it and smash it to pieces. (or just knock it over, either way, disabling it)
Point is, the one phrase of "looks like small arms fire... with some explosives damage, probably seismic charges" is hardly enough to go on for making a rock-solid argument that there wasn't a single piece of military hardware on the colony.