What's beautiful, when does ugly stop? As far as I am concerned, overbore is when you reach the point of diminishing returns. If 60 grains of a certain powder gives you 3,000fps and 89 grains, in a larger case gives you 3,050fps, then you have slightly more speed, and substantially less barrel life. Everybody may have their own personal yardstick of what is reasonable and practical, and it changes, depending on the application.
I am absolutely not an expert, but this is what I thought I herd. That overbore is sometimes used to describe a case that has more capacity than bullet weight, I thought I heard that somewhere. But ther are some instances that overbore does work better than expected. The STW case is way over bore but somehow it works for it and produced very high velocities taking agvantage of its power charge. Now I think the Lazzeroni long maginum cartridge in 257 is larger case capacity than the STW but doesn not produce nearly as high velocities for its case size Based on ifo on their site.
I saw a wildcat book that showed a Picture of a 17/50BMG NOW THATS OVERBORE, but its so cool looking. Ask it over on Varmint Hunter