Currently both of my guns have anti-inhale devices on them. Granted this is a bit tougher to do with large bore metal tubes.
For PVC however it is dead easy.
You just need a coupling in the same size as your tube, cut off a small piece of pipe, like 1/8th of an inch.
Take the mouthpiece end of your pipe, and slice off a little of the inside of the pipe.
I cut my coupling in half (thus getting 2 mouthpieces) glue the sliver you cut off into the bottom and glue it onto the pipe.
When you shove a dart into the pipe, the cone will slip past that small slice of pipe you cut off. And catch in the groove that is formed when you trimmed the end of the pipe.
This whole process takes longer to type than it does to do, especially if you have a good pvc cutter.