(is it possible to just do a search&replace on the map files...?)
basically, no.
it happens because Line of Sight starts from a different voxel than Line of Fire (LoF is ~4 voxels lower, iirc)
so, re. fences, some you can see through but might not be able to shoot through, or vice versa. (depending on the z-voxel of the slats or chain-links, etc)
truly "fixing" it to be more human-like (us humans can shift our perspective, or the barrel of a gun, a few inches no problemo) would require changing the OxC sourcecode -- but voxel-tracing in code is substantially more complicated ...
edit: there may be workarounds for modders, like uh perhaps removing their LoFTemps and setting their type as Content w/ an appropriate bigwall var, but even that wouldn't really work since the fence couldn't be shot away and walked through anymore ...
edit2: a possibility might be to add an MCD-variable "shootThroughable" ...