To get a good purr, I have to cut down the tape on my diaphragms - not much, maybe 1/16".

Just get it cut back to the point where you can slide it an extra 1/4" deeper into your mouth without gagging.

It takes hours and hours of practice, but you'll just need to roll your tongue against the call and the roof of your mouth.

It's not exactly the same as if you were to make a "Rolling RRRRR" sound (without the call in), but it's really close.

The key for me was cutting the tape, but don't cut it too much, or you won't get a good seal around the back, and it will just sit loose on top of your tongue, with air passing over both sides. It has to be a perfect fit. Try this first with some old calls that you're ready to scrap.

If you can master the purr, you can move on to the fighting purr, which is much more aggressive with clucks and putts interspersed in the sequence. After that, you can start to master the gobble, which is darn near impossible.