Normally, called shots are full round actions, Improved Called Shot lets you make one as a standard action, and Greater Called Shot says:
Whenever you make an attack, you can choose to replace that attack with a called shot. You can make multiple called shots in a single round. Each additional called shot after the first made in the same round takes a –5 penalty. In addition, a called shot that deals half the creature’s hit points of damage (minimum 40) is a debilitating blow.
So, when I charge, can I replace the charging single attack with a called shot and still gain all the other benefits of charging?
Charge rule:
After moving, you may make a single melee attack. You get a +2 bonus on the attack roll and take a –2 penalty to your AC until the start of your next turn.
