Really hard to come up with and develop a good new wildcat round since all of the good ones have been taken.
During the early 60s a gentleman named Clerke (pronounced Clark) came up with what I saw as a pretty cool round called the .38-45 AUTO. I have a round around here somewhere. There was no shortage of 1911 pistols around in both 45 ACP and the 38 Super round. Clerke necked down a .45 ACP to a .357 case mouth. RCBS still makes the case forming loading dies
which can be seen here. Hot little round. The shooter would just do a barrel change on their 45 ACP 1911 using a 38 Super barrel reamed for the new cartridge. Clymer made the reamers. It was a good example of adapting something out there to a new cartridge. The biggest downside to the round was after necking down the 45 ACP you would get around 4 firings and the case necks would split. None the less a hot round for its day. So really when Sig necked down the 40 to develop the 357 Sig it wasn't a new concept or idea. The Clerke round went by a few names, I used the name that RCBS does for the dies.
Years ago I had a strong fascination with the 7.62 X 25 Russian Tokarev as a hot little round pushing 30 caliber bullets.
Ruger has the Blackhawk chambered in the 30 M1 Carbine round. I once had an idea as to necking down a .357 Magnum cartridge to a .30 caliber. That was about 20 years ago. Actually designed the cartridge and had drawings made up. Checked with Clymer for reamers and RCBS for dies. I forget the cost but as I recall it wasn't that bad. The idea was to get an extra cylinder from Ruger for their Blackhawk revolver chambered in 30 Carbine. Unfortunately Ruger won't sell their cylinders and they need fitted by Ruger. So a revolver would need sent to Ruger explaining the cylinder was lost. When the new cylinder was returned in the gun the old cylinder would be reamed for the new cartridge.
In theory it would have been one hot revolver round. I still have a few proto type rounds I made up years ago.
Life got busy and I never pursued it. I wasn't sure how much pressure I could get away with in the Ruger cylinder and frame. The merit was that like the Clerke round most of what was needed was already out there. Someday the old drawings will turn up and who knows, maybe a renewed interest. Wish I hadn't sold the revolver.
Ron