Skill Correct Behavior:
The arm cannon damage should be in 2 parts.
If the player uses Holy Property with Holy Cannon Ball, the damage should be more greater that using only Neutral Weapon + Holy Cannon Ball. This apply also on Shadow enchant Weapon + Dark Cannon Ball.
Also if the enemy the player uses Elemantal Enchant Weapon + Soul(Ghost) Cannon Ball, the damage should be greater on Ghost Property enemy comparing to Neutral Weapon + Soul(Ghost) Cannon Ball
Current Behavior of Arm Cannon is same like Cart Cannon base on the test conducted.
Cart Cannon behavior is when using an elemental Cannon Ball, it takes the property from the equipped Cannon Ball and then reapplies its modifier on the whole damage" (excerpt. irowiki)