I originally turned an o-ring groove into my inserts because I was using a loose 5/8" bit.
Then, I had this guy named Al send me a 5/8" bit that wouldnt' fit into his drilling plans, and I've been turning tight friction-fitting duck calls ever since.
Open water inserts are JUST slightly smaller than timber inserts when it comes to friction fitting.
My customers have been happy with the friction fit, and I have heard of no cracked barrels or anything like that.
The inserts also have a groove cut into them for the double lanyard like Braz was talking about.
Parker