I have three of the WFT's , only one takes the inserts. Ian, it's not rocket science and certainly not necessarily a "precise" exercise. Case lengths are usually listed as within a acceptable range of length, and that's what I shoot for. I trimmed some 223 today with the WFT and they were ALL within .0015, way more precise than I'd expected. I got the insert removed and took all the "generic" measurements needed to cut to size, but the tricky measurements will be for cutting the ID to exacting size for whichever caliber I choose. Saw a utube video of a insert made with a 3D printer. Looked good and worked great and the color was Navy Blue.