The description is pretty vague so it's hard to figure out what it is unless you really remember that particular scene very well.
It would help out a lot if you could remember some technical specifics so we can at least determine the gameplay genre, the setting and maybe the point of view (first or third person).
So you mention you need to get some items and investigate something. I'm assuming that's just the mission objectives, but were there any gameplay mechanics involved in that, as in using some scanning tool or a special detective vision to find this item or to investigate the surroundings? Or was it just walking around, entering rooms? Were there any interiors at all?
You also mentioned a field full of barns, so I'm at least guessing the game wasn't sci-fi, no cool futuristic tech, shields, laser weapons? It was probably set in the present, or was it maybe some past point in history? Or maybe a fantasy setting (but I'm assuming not if there were firearms)?
Since there is an ambush occurring, was there any shooting involved, did you have a pistol, machine gun or something like that to shoot back?
It would help a lot if you could remember if it was 1st or 3rd person, were there any other gameplay mechanics involved, sneaking, talking to NPCs with or without dialogue choices, an inventory screen etc?