For an airgun project I'm working on I had to cut a 5/16" radius in a piece of steel.
I made a simple boring bar. There's a cross hole reamed to 3/16", a #10-32 setscrew in the center and a short bit made fron a broken end mill. I set the tool to cut the 5/16" radius. (3/8" bar diameter+1/8" tool sticking out of bar, for a circular sweep of 5/8")
I did a test piece in aluminum, feeding in then down, progressively.
Radius gage fits perfectly.