Those rule come into play with COD (but change in GIA)...
107.2 The “Gun” target classification on the
TO HIT Table assumes that the weapon has
been properly dug in or emplaced (48.2).
Anytime a gun of any type starts a scenario in
transit or moves during the course of a scenario
it forfeits its “Gun” target classification on the
TO HIT Table and assumes “Vehicle” target
status with a +1 target size DRM (64.6).
Exception: large guns with a vehicle target
equivalent have no size DRMs.
107.21 Guns with a vehicle target
classification which have been moved can claim
“hull down” status by setting up directly behind
a wall, roadblock, wreck, crest (77.1) or other
such obstacle.
107.22 Once moved, a non-vehicle sized gun
may regain its “Gun” target classification only
if its crew makes a successful entrenchment
(54.2) dice roll although no actual entrenchment
counter or benefits accrue. The “entrenchment”
attempt could even take place in a building hex.