Called shot arm does stack and a succesful hit last four rounds.
Jacc uses called shot and one of the benefits is you dont loose other attacks in the round. So you could actually get 4 called shots in one round.
It is quite a useful feat if you have enough AB to make it hit.
One thing people tend to forget is that this is not a ranged specific feat, called shot can also be used by melee fighters, I guess the name is slightly confusing.