Slip leads are often used very effectively together with gun dogs.
Slip leads can be used as both collar and lead i.e. the dog should not have a collar on. A lot of dogs are hunted without collar. They can be used to help correct a dog that is pulling on the lead.
I purchased this lead on line from Sporting Outback Supplies (SOS). If you live in this part of the world I found this company to provide good service.