To the OP ….. for field training, back up and think of the dog as a young pup and introduce the training just as you would for the younger dog.

Carol Cassity has a great book showing drills for training and they are marked as being for a dog just starting out, a dog in transitional training and drills for the advanced dog. This book can be found in Canada here - https://store.dakotacreekretrievers....ev_p/cc002.htm

As others have said, take the gun introduction slow …. Have always found the best way is to start at a distance and slowly decrease the distance between the gunshot and the dog BUT do this introduction with birds …. Preferably pigeons with flight feathers pulled so the dog is SO excited and bird crazy it doesn’t even hear the shots in the distance.