It is a problem with the fire and geometry LODs of the bunker, the AI thinks there is a big wall that they can't shoot through in front of them. Try to find a bunker with out the problem, or an addon that has the gun allready in the bunker... and SpecOps: the only reason AI can shoot over "dead" sandbags is because when the collapse the fire LOD becomes smaller, doing that to the bunker would cause it t collapse as well, and it's not a pretty sight...