Earlier comparison with mouth call has some merit to me. The lips have to be loose enough to vibrate for rasp. Learn to produce that without aid of saliva or candy or whatever and it is more reliable in hunting situations in my opinion. One thing that seems to help me is opening mouth a little more to give space between upper and lower teeth for relaxed lips to have room to vibrate. Seems for me I like end of mp to reach about where inside moist part of lips meets the drier part that rests on other lip.
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