What is the requirement to destroy shield?

Solution 1:

UPDATE

With the release of Ultra Strikes, new tactic to destroy portal's shields and other mods emerged. If you stand right on the center of a portal and fire those, there is a very high chance to destroy the mods.

UPDATE 2

New shield stickiness was introduced recently.

Shield Type         Old   New
Common shield       30    0
Rare shield         40    150000
Very Rare shield    60    450000
Very Rare "AXA"     70    800000

The tactics for destroying mods with ultrastrikes remain same, however it is now much harder to destroy the vrare+ shields.

OLD ANSWER:

The mechanics behind destroying portal shields changed several times, but this is probably closest to the reality now. If I recall correctly:

• Common shield: 10% chance
• Rare Shield: 8% chance
• Very Rare Shield: 6% chance

Also there is a difference between firing a L1 and L8 burster. The percentage might be influenced by the damage potencial of the weapon.

There used to be a special way to approach a portal from distance, destroying all it's shields without taking a damage and then dealing with unshielded portal. It utilized these factors:
1. Portal hit range being around 60 meters.
2. XMP L5 having range 90 meters.
3. Fixed probability of destroying a shield.
4. Firing from range between 70 - 90 meters.
But this now doesn't work as there are more factors influencing the destruction of a shield.