Long Technical Answer.
When building a scenario you create an Ammo level setting for each side. In your scenario lets say there are 500 Union shells and 350 Rebel shells.
Next...
There are two different ways Artillery Ammo can be used in the optional rules. If you have Artillery Ammo by Cannon selected than it will double the ammo to 1000/700. The reason is that each cannon will fire one shot. So a 6-gun Union battery will use 6 shells. Leaving the Yanks with 994 shells. A single Rebel gun firing will leave the Rebs with 699 shells.
If the option is not selected it will then start you with the default level of 500/350. When the same 6 Union guns fire it will cost them one round leaving them 499 left. The single Reb gun will fire and cost you as many shells as the Union did though they fired 5 more guns. After both batteries fired it would be 499/349 left despite the Yanks firing 5 more guns.
Thats the techincal answer at least.
Why are ammo levels set to what they are? Now that one is something I am not sure of. I am sure they use some sort of measuring stick but I couldn't tell you for sure. Be happy the ammo levels or low though. Otherwise those gun-happy Yanks would just try to fight all their battles rolling cannon forward like some giant tank. Yank infantry ain't worth spit
