I have tried player Sniper vs AI sniper as well as a sniper on your team set to engage at will. I think there is a Value target Order it does.
I think (I maybe wrong) Order is:
Anythreat that is Shooting at Sniper.
M2 machine gun
Officer
Machine Gunner or Medic (depending??)
RPG / LAW.
Ect....
They kind of have a mind of there own.
I think there is some Search threads on this and Some Scripts in Editors Depot. That my help out.
Maybe try a E. Loon shooting Gallery.
Set up E. Loons with no magazines in weapons, ranging from Officer to Plain Soldier set to aware and patroling. You are separate from your Side sniper and see what he does.
Also Distance does matter. Too short and Too Long affects the shooting. Look into set View-distance and maybe a spotter with him.
Hope this helps