[Rick Parker]

Flow dynamics is the key here. The tapering (on all jets) is intended to reduce or limit the turbulance created by the restriction. The "hole size" will determine the ultimate flow rate, I think most jets are actually reamed to size. SO........if you have an idea of what size you want, the chapest way out is to find an Industrial Supply house near you and purchase some specific lettered or numbered drill bit(s). Individually they are very inexpensive and are intended to be used in a pin vise. Years ago when I was using DCOE's and when changing cams, pistons, etc, on the cheap we would carefully solder the hole in the jet and carfully drill it with another size bit and then carefully relieve the sharp edge of the hole with a larger bit or very small countersink. We would also run a bit down the interior to remove any excess solder.
This allowed us to get the jetting very close before purchasing the final machined jets, since we didn't have an assortment to draw from. It seemed to work well; that was a long time ago. I would do it again too.