What you are thinking of is the WZL barrel made by On Target tech. Its a rimfire that is offset in the bore so that the centerfire pin hits the rim of a rimfire. Pretty stupid simple, but pretty neat too. They also do an over under rimfire for contenders, since you can flip the firing pin location on those, using the same concept.
http://www.ottllc.com/
They are doing some really inovative stuff with barrel making. I still have yet to purchase one of their barrels, but I am thinking this summer might be the right time. They acctually cut their chambers with electricity instead of drills. Absolutley exact tolerences. Check out the centerfire sub-calibers at the bottom based on the 5.7x28 case. A .14cal included, something I have always fancied messing with.