This biggest problem I have with using any existing GV in a long-range artillery role is that the gun does not remain trained out at the same az and el; every time you fire, the barrel ends up moved. There used to be a little cheat you could use to quickly return to where you were pointed before you pulled the trigger, but it seems to be no longer be available. So, even with a spotter, there's no way to fire accurately from beyond line-of-sight. Even with LOS, it's difficult at long range.
At any range up to 6k I can put round after round within fifty feet of any target from an M4A3-75. At 10k range I can do the same thing with a little elevation advantage. I do not need a spotter in the majority of situations. The longest range vulch of an aircraft I have ever killed was 9.9k.
Inside of 400 yds the M4A3-75 can kill a TigerI in one shot even with HE. It's all about knowing how, or putting a little thought into it to figure it out.
You can ask Tyfoo about that because he was witness to the closing of his field for a six hour period last week. When Rooks started to put pressure on our base I killed the barracks on V88 (OZKansas) without ever leaving V103, and I did it with less than fifteen shots.
Gamer (the OP in this thread) has been witness to my doing this in the earlier war arenas. He has not figured it out yet, which is why he wants artillery. You already have the best artillery you can have and you don't know how to use it!
Just think of it this way. If you get a howitzer, then it will be killed the same way field guns are now. Keep your M4s handy.