Muddyboot, I used common .452" 230 cast RN bullets to start, then switched to .451" 230 grain plated bullets in my Webley for easier clean up. I wondered if the old gun would even stay on the paper at 15 yards, but it did much better than I expected.
FWIW, I slugged a chamber throat and the barrel of my gun. The chamber throat was .450". I couldn't accurately measure the barrel due to the 7-groove rifling, but the largest diameter I could measure was .450". Perhaps why my .451" and .452" bullets seem to shoot just fine.
ETA: Ref a source for the ring, someone on this, or another forum, referred me to a seller. I can't recall the seller right now, but an internet search would probably turn up a source. I just checked Ebay, and saw a different, and not inexpensive, type ring from Canada, but looks to me like it would work...
OK, Think I found where I bought my conversion ring; partsforantiqueguns.com in Georgia.