This thank you goes out to Texasmac for measuring some .50-70 3 groove slugs for me. They came from a 1868 TD, a converted 1863 Sharps carbine and an 1871 Springfield rolling block. Several surprises. Of the three, only the TD had a .515 groove which is what I expected. The Sharps is around .510. I assumed that it was a government .515 reline from way back when, but now I'm thinking it's more recent, for .512 bullets. The roller was the biggest surprise with a .522 groove! It shot ok with .515s but I'm going to try the fattest bullet I can squeeze into a case to see if there's an improvement.