Slitherine

If shooting at a battle group that has mixed armour classes, the shooters’ POAs are determined not by the armour class of the targeted base, but as follows:
 Unless the shooting base is entirely behind a straight line extending the target battle group’s rear edge, by the armour class of the front rank of the target battle group. If that has mixed armour classes, by the majority type in the front rank e.g. when shooting at a pike and shot battle group with a single file of armoured pike flanked by files of unarmoured shot on each side, the target counts as unarmoured. If there are equal numbers of bases with different armour classes in the front rank, the target counts as having the lowest of these armour classes. Exception: When shooting at the front edge of a battle group in Swedish brigade formation, the shooters’ POAs are determined by the armour class of the front rank pike base.
 When the shooting base is entirely behind a straight line extending the target battle group’s rear edge, use the rearmost rank’s armour class (determined as above) to determine POAs.

rbodleyscott wrote:If the target BG is entirely armoured, muskets shoot at a - POA at long range.

However:

If shooting at a battle group that has mixed armour classes, the shooters’ POAs are determined not by the armour class of the targeted base, but as follows:
 Unless the shooting base is entirely behind a straight line extending the target battle group’s rear edge, by the armour class of the front rank of the target battle group. If that has mixed armour classes, by the majority type in the front rank e.g. when shooting at a pike and shot battle group with a single file of armoured pike flanked by files of unarmoured shot on each side, the target counts as unarmoured. If there are equal numbers of bases with different armour classes in the front rank, the target counts as having the lowest of these armour classes. Exception: When shooting at the front edge of a battle group in Swedish brigade formation, the shooters’ POAs are determined by the armour class of the front rank pike base.
 When the shooting base is entirely behind a straight line extending the target battle group’s rear edge, use the rearmost rank’s armour class (determined as above) to determine POAs.

So for normal 4 shot, 2 pike BGs, the armour of the pikemen does not help against shooting.

The main beneficiaries of the rule are 16thC pike keils and Swedish brigades.