I disagree. I've tested this many times, always with the same result. I put a battery of napoleons and a battery of 3 inchers on the Kansas map about 800 yds apart and have them go at it. After 50-60 rounds/gun, (solid shot), all the napoleons are either destroyed, routed due to casualties or have their caissons blown up. The 3 inch guns lose 1 or 2 guns in that time. I agree that it is probably too small a destruction rate, given the historic accuracy of the artillery, hence my suggestion that the target size be increased.So I would suggest that guns take not only a larger moral hit but also take more casualties, because they really don't take any at present.
I don't agree that we need a special morale lose rate for the artillery. A man is a man, whether he carries a rifle or a rammer. Norb is correct in modeling fatigue in the game as he does. If the morale hit is too low for arty fire on a cannon crew, then it is also too low for the infantry and cavalry. That can be modded to the level the player feels is appropriate.
One morale change I do feel is appropriate is that the loss of a gun due to counter battery fire has a large morale impact on the rest of the battery. That is supported by the historical record. But as NY Cavalry points out, that may require a good bit of coding.