Originally Posted by SKI008
I think .675 is probably cutting it too low. The window according to cockerpunk's youtube test is .006-.008 max underbore before things start getting bad but I think if you can do .002-.004 you're on the money.
Things don't really get "bad" they just get "suboptimal" ... breakage and such doesn't really happen, even in those tests the increase in breakage was tiny. Fact is underboring really doesn't cause ball breakage per se ... the ball will actually get stuck in the barrel before it breaks most times. The exception to this is abnormally brittle/uneven shells, such as in very cold weather.
That said, you do probably want to aim for .008-.010 or under. Of course this is all dependent on how you measure your paint ... there is no standard way to measure it.