LSC, I really only make goose calls. The only mandrel that I can not do without is a 5/8 expanding. Wade carpenter has them with a taper, I believe. You will drill your insert and barrel to 5/8. You could do 3/4 for the barrel, but it will make the call harder to blow. My advice would be to drill your barrel 7/8 to just beyond a sr-1 gut from wade and then 5/8 the rest of the way. That is a good place to start, because it will give you a good sounding and easy playing call. From there you can start experimenting. The above mentioned dimensions should be easier than doing three steps in the barrel. After drilling, place the barrel on the 3/4 to 1 inch of 5/8 bore you have left, then bring a live center up to the 7/8 part for stability. Then turn away. The insert is much more simple, because it is mostly 5/8. You will need a 3/4 step for the gutsert as mentioned before. Hopefully that helps. Short answer is 5/8 mandrel.