Cut to the chase and measure the throat diameter, that is what drives proper bullet diameter not groove diameter. If doing a chamber cast to include throat diameter isn't possible, a pound cast certainly should be. Google it. I bet by now there's a YouTube video since it's been in the bullet caster's lexicon for years now.

"You can lead a man to logic, but you cannot make him think." Joe Harz
"Always certain, often right." Keith McCafferty