There were many conversions of the 30 carbine to different calibers. The most common was the .22 by Melvin Johnson who had tried to sell it to the US military. They liked the idea but needed a heavier bullet and higher velocity but with the .30 carbine case it could not be done. Melvin Johnson who designed the Johnson automatic rifle keep the idea going by making conversions available and even offered his own line of factory ammo and reloading equipment. It was not a great success. Even today if one is found at a dealer it will most likely sit on his self for sale for a long period of time. The last one I saw along with a box of factory ammo, fired cases and dies sat on a dealers shelf for over a year and a half. He kept dropping the price and finally got rid of it is a large trade with another dealer.
Oh and there is ammo floating around for the 4.6x30 as designed by HK. as stated above the pressure is far to high for the carbine to handle safely. at least 3 companies in Europe have made this ammo.