That is so yesterday.
It's been proven time and again that what is most important for cast bullet accuracy isn't groove diameter, rather throat diameter is the key. Size around .0005" under throat diameter for a perfect fit that guides the bullet straight into the rifling. An added benefit is less chance of leading (less blow by of hot gasses), and being able to use softer alloys than is intuitive. A Cerrosafe chamber cast to include an inch of the rifling will tell you more than pounding a lead slug through the bore. An alternative means is to do a pound cast- a lead filled empty case or a case with a brass rod fitted so as to fill the neck (as an anvil, so to speak) to support a soft lead slug in the throat which is then upset to fill the throat and leade by whacking it a couple times with a steel rod from the muzzle that closely fits bore diameter (not groove diameter obviously). The resulting slug is a perfect "negative" of the throat dimensions. It can be carefully stored for all eternity and used as a reference later on, something you can't do with a Cerrosafe casting the ephemeral nature of which only lends itself to a narrow window of opportunity in which to take the measurements.
Further pursuit of cast bullet accuracy will lead you farther down the rabbit hole. The next step is to configure the bullet nose to fit the leade. One can re-throat one's rifle with a throating reamer to give a gentler taper to the leade, and then make a die with the same reamer in which to swage the nose of the bullet for an absolutely perfect fit- a tapered plug fitted to a tapered hole so to speak. All that is a lot of work, true, but again its all about starting that poor soft bullet as straight as possible into the rifling. By pitching headlong down the above mentioned rabbit hole (and paying heed to other protocols too) there is no reason why one can't achieve the accuracy one gets with jacketed bullets.
