I agree with everything EODLT says with one exception... When you go into the field with her, let someone else go with you to fire the capgun after you get her into a big ole game of play, praise, praise.....As she is retrieving, have him turn hisback to you and the dog before he fires.Then step up to 22 rifle, etc., depending on the outcome. I worked with a profession trainer and this was his method.This worked for me.
Now everytime I pick up a watergun, my lab gets all excited thinking we gonna kill something.
Good Luck!!