I think any gun may be placed in a factory location but I can´t find the rule.
I do not think that applies in general - only in certain HASL (RB e.g.) rules.
C2.7:
2.7 PROHIBITED HEXES: A Gun cannot occupy an upper building level
[EXC: Fortified Buildings and mortars on Rooftops], nor can it occupy a Water Obstacle, crag
[EXC: mortars; B17.4], marsh, or Irrigated-paddy (G8.12) unless dm and possessed or in/on a vehicle/boat. Small-Target-Size Guns and AT/INF Guns that are not large targets are the only ⅝" non-vehicular Gun counters that may ever occupy a building/rubble hex
[EXC: Rooftop mortars (B23.85); Fortified Building (B23.93)].