Back in the mid-seventies I experimented around with making .243 bullets out of .22 Magnum cases. I had noticed that .22 Mag cases were 6mm in outside diameter. It sorta worked, but I just did it to see if it could be done --a "proof of concept" kind of thing. I'm sorta of the opinion that the art and science of firearms should not reside solely with big manufacturers and governments.
I made maybe 10 or 15 experimental bullets all in all and they worked moderately well. The process I used was subject to considerable improvement, but I had made my own dies, and having proved the feasibility, I went on to other things.
When I got rid of my lathe and whatnot, I turned over a bunch of miscellaneous tools and dies that I had made to my son, including a tool bit for converting .357 cases to .38 Super, some other crap, plus the dies I had made for the .243 bullets.
He came across those dies about three years ago and I told him what they were for and how they worked, and he decided to see if he could make .223 bullets out of .22 LR cases. He's got his own machine shop now, so I told him how to make D-reamers for the dies and offered some other suggestions.
This was before either of us realized that there were others doing the same thing. Phooey.
So anyhow, he's almost perfected the process now, and here's the result. He's having the same problem I had, which was blending the ogive seamlessly into the bearing surface of the bullets.
However, he's gotten 2 1/2 inch groups out of these 52 grain bullets, so I guess that little shoulder doesn't matter that much, and "improvements" will be more the result of refining the "art" and technique rather than in refining the process itself.
In any case, he's also proved the concept.
Terry, 230RN