I looked through the loot spawn and found out exactly what to look for to find top-tier stuff. There's 2 higher tier item spawn types, Military and MilitarySpecial. Military spawns most of the good stuff and is found everywhere. Hangers, Control Towers, Deer Hides, tents, and those fire station things are all Military. MilitarySpecial is only found in one building currently and spawns most of the Military stuff along with top-tier stuff like the M107, Night Vision Goggles, and the machine guns.
The only building that currently spawns MilitarySpecial loot is "Land_Mil_Barracks_i".
Here's what it looks like. Ignore the buildings in the background, I spawned the building at the NE Chernarus airport in single player so I could get a good look at it.
Here's how the loot compares between Military and MilitarySpecial
Military Only
M9 (unsilenced), MP5A5, MP5SD, Entrenching Tool, Sandbag
MilitarySpecial Only
M249, M136, SVD, M107, M240, M4A1 AIM SD, M16A4 ACG, M4A1 HWS GL Camo, Mk 48, Pipebomb, Night Vision Goggles, Rangefinder Binoculars
Both
M16A2, M16A2 GL, M4A1, M4A1 AIM, M9SD, AK47M, AK74, AKS74 Kobra, AKS74 U, M24, M1014, DMR, M14, PDR, Remington 870, Glock 17, M4A3 CCO, Binoculars, GPS, Military Flashlight, Coyote Backpack
Both will also spawn the random junk that's found everywhere, medical supplies, and assorted ammunition.
I don't yet know where any Land_Mil_Barracks_i are on the map and it's likely that they're spawned via the mission file, which I don't have.