4th Edition DF chart - Opportunity Fire question

[plloyd1010]

I'm thinking probably no +1 for Artillery on the assumption that the bonus on artillery assumes they are static and cannot take as much cover as other units. When towed, opp fire is similar to that on other moving units, so +1 on that only.

That is sort of my thinking. Personnel units are likely crouching or in minimal cover when stationary (in the absence of anything substantial). When moving they lose that small advantage. Gun crews and ammo handlers are somewhat more upright in their jobs, thus their exposure would change less. Some of that is obviously rationalizing, but it works.