After a few rounds of bouncing bursts off it's hardened armor they switched to attempting Bulls-Eye double-tap APDS shots which worked out a bit better (care to expound on the math Herr?)
Sure. I'm assuming average resistance rolls of Body 9 + AV/3 to cut down on the variables. Obviously, better or worse rolls means more or less hits are needed to damage the spirit.
Force 9 Spirit (Body 9, modified AV16) vs Assault Rifle with regular rounds (DV11, AP-2); needs 6 net hits to both exceed AV and deal any damage at all (0.33 unresisted DV on average)
Force 9 Spirit (Body 9, modified AV12) vs Assault Rifle with APDS rounds (DV11, AP-6); needs 2 net hits to exceed AV and 3 net hits to reliably deal any damage (0.67 unresisted DV on average)
Force 9 Spirit (Body 9, modified AV8) vs Assault Rifle with APDS rounds and Bulls-Eye Burst (DV11, AP-10); only needs 1 net hit to both exceed AV and deal damage (2 unresisted DV on average)
And just for kicks...
Force 9 Spirit (Body 9, modified AV11) vs EBR with APDS rounds (DV12, AP-7); only needs 1 net hits to both AV and 2 net hits to reliably deal damage (1 unresisted DV on average)
Force 9 Spirit (Body 9, modified AV5) vs EBR with APDS rounds and Bulls-Eye Burst (DV12, AP-13); only needs 1 net hit to both exceed AV and reliably deal damage (5 unresisted DV on average)
As you can tell, using APDS rounds with the Bulls-Eye Burst called shot you only need a single net hit to start damaging the spirit if he rolls completely average. Compare this to the 5 net hits you'd need using just APDS to do similar damage.
Statistically, it's worth pointing out that the -4 dice will on average only result in 1.33 net hits, so you're by far better off taking the lower dice pool over hoping to score significantly more hits without the called shot. Granted, this is all in a math-hammer exercise in a vacuum where the damage resistance roll is constant, but at least it gives you a rough guideline of what to expect based on probabilities.