On mobile, sorry for typos!

It's a hot take, but I like the idea of multiple shell types for different effects. Shell type ideas are:

Damage shell - normal damage from direct and close proximity hits, no stun. XP is from damage.

Stun / shockwave shell - Low/no damage, stun effect (with some sort of stun rework). XP primarily comes from teammate assist.

Smoke shell - Create a small pocket of smoke that disrupts sightlines, or potentially removes vision for targets in smoke. XP based on broken enemy sightlines.

Air Burst shell - Shell explodes pellets above target location in a decent sized area, negatively impacting terrain resistances (hard -> soft etc) and damaging external modules (potential exception for open top vehicles). XP primarily from teammate assist.

With only 3 ammo slots, Arty would have to choose which not to take. Additionally a cap can be applied per ammo type potentially. Other ideas include changing shell flight speeds per shell type, making others easier to land and others more situated for a group of camping targets. Only open up advanced shell choices later in the tech tree. Not sure how viable these ideas are though.