I took the most ammo-efficient caliber of artillery for example - 300mm, and numbers are:
OFFENSIVE VALUES:
- Regular Arty (300mm) 1200 FP -> 300 soft atack (divisor of 4 for ranged) for 54 ammo = 5.5 attack/ammo
- Rocket Arty (Small Rockets) 4000 FP -> 1000 soft attack for for 66 ammo = 15.2 attack/ammo
- Rocket Arty (Big Rockets) 6000 FP -> 1500 soft attack for 174 ammo = 8.6 attack/ammo
- 500kg bombload: 125 FP -> 125 soft attack for 12.5 ammo = 10.0 attack/ammo
- 3000kg bombload: 500 FP -> 500 soft attack for 75 ammo = 6.7 attack/ammo
- 10k bombload: 1500 FP -> 1500 soft attack for 250 ammo = 6.0 attack/ammo
The example only show offensive values, in defence situation is somewhat better but artillery would still be in a questionable position (worse than rocket artillery with small rockets on offensive by ~30-40%).
Given that artillery ammo consumption is tied to general Gun ammo consumption (e. g. for tanks), I suggest changing Ranged Indirect Soft Attack divisor from 4 to 3 or even to 2, reducing firepower of other indirect weapons accordingly to maintain their old characteristics while artillery would receive the most direct buff. Other way could be increasing ammo consumption of all the other indirect fire support (rocket arty, tac missiles, plane bombs, etc.).