@Buddys:
Please run from any shock collar user! I just recently was hired by a couple who adopted a black lab puppy, at 4 months old he was overly nipping and growly. They had this 'trainer' put on a collar and poor Jasper got worse.
Unfortunately there aren't a lot of trainers who use the collars effectively, but someone who is skilled can work magic (I have seen it done). They get results by very subtle corrections, not full on jolts. But I would concur that it is a last resort, if you can't solve a problem any other way. (I know of a couple of dogs who did not get put down because a skilled trainer reformed them. However, the owner must also learn the correct use of the collar for any ongoing concerns, and it is more complicated than just buying one and slapping it on the dog.)
On leash aggression isn't unusual, and can be challenging to deal with. Often a dog is "good" at classes and shows, not so much when walked locally. I know of two titled Utility Obedience dogs that are wonderful at shows but dog aggressive on walks. Go figure…..